MA. 


SAUNDERS. 

Medical    Books. 

(3  S.  i..th  St. 

Phila. 


THE  LIBRARY 

OF 
THE  UNIVERSITY 

OF  CALIFORNIA 

PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


MANUALS 


STUDENTS  OF  MEDICINE. 


COMPARATIVE  ANATOMY 


AND 


PHYSIOLOGY. 


P.  JEPFEEY  BELL,  M.A., 

PROFESSOR    OF    COMPARATIVE    ANATOMY    AT    KING'S    COLLEGE. 


ILLUSTRATED    WITH    229    ENGRAVINGS. 


PHILADELPHIA  : 

LEA     BROTHERS     &     CO. 

(LATE  HENRY  C.  LEA'S  SON  &  Co.) 

1885. 


SEPTIMUS  W.  SIBLEY,  F.R.CS., 

AS   A   LITTLE   TOKEN    OF   RESPECT 

FOll  THE 

SKILL   AND    SYMPATHY   WITH   WHICH   HE    EXERCISES 
HIS   BENEFICENT   ART. 


PEE  FACE. 


THE  reader  who  is  sufficiently  acquainted  with  the 
progress  in  vertebrate  physiology  during  the  last 
phase  of  physiological  methods,  and  who  knows  how 
scattered  and  incomplete  are  the  investigations  which 
have  been  made  by  the  same  kind  of  physical  and 
chemical  inquiries  on  invertebrate  animals,  will  not 
expect  to  find  in  the  present  volume  any  complete 
statement  of  the  physiology  of  animals,  in  the  sense 
in  which  that  term  is  now  used.  Such  observations 
as  have  been  made  without  especial  reference  to  the 
vital  processes  of  man  are,  for  the  most  part,  very 
valuable  and  suggestive  }  but  the  time  to  write  a  text- 
book of  Comparative  Physiology,  as  we  now  understand 
it,  has  not  yet  arrived. 

All  that  I  have  attempted  to  do  in  this  little  book 
has  been  to  illustrate  the  details  of  structure  by  a 
notice  of  such  experimental  inquiries  as  I  have  con- 
vinced myself,  or  have  adequate  reason  to  believe,  are, 
in  their  broad  outlines,  correctly  stated.  I  have  much 
more  attempted  to  make  use  of  what  were  long  since 
called  the  experiments  that  Nature  makes  for  us,  by 


mi    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

referring  to,  sometimes  perhaps  insisting  on,  the  dif- 
ferent methods  by  which  similar  results  are  attained  by 
different  animals.  That  which  I  have  most  constantly 
kept  before  myself,  and  which  I  hope  the  student 
will  faithfully  bear  in  mind,  is,  that  there  has  been  an 
evolution  of  organs  as  well  as  of  animals,  and  that  he 
who  desires  to  understand  the  most  complicated  organs 
must  first  know  the  structure  of  such  as  are  more 
simply  constituted. 

In  pursuit  of  this  object,  I  have  written  about 
organs  rather  than  about  groups  of  animals ;  but  I 
have  added  an  index  in  which  the  various  parts  of 
an  animal  are  collected  under  the  head  of  its  name ; 
so  that  the  student  who  desires  to  use  this  manual  as 
a  zoological  text-book  will  have  no  difficulty  in 
selecting  the  portions  of  the  chapters  which  bear  on  a 
particular  form  or  set  of  forms. 

I  have  departed  a  little  from  the  ordinary  method 
of  writing  a  handbook,  in  somewhat  plentifully  inter- 
spersing the  names  of  my  authorities  for  various 
statements.  I  have  done  this,  not  only  because  it 
recommends  itself  to  my  sense  of  justice,  but  becau.se 
zoological  science  is  just  now  advancing  so  rapidly 
that  many  observations  and  suggestions  have  to  be 
incorporated,  even  in  a  text-book,  before  they  become 
the  general  property  of  zoological  workers.  My 
indebtedness  to  the  personal  teaching  and  the  pub- 
lished writings  of  Professor  Ray  Lankester  must  be 


PREFACE.  ix 

by  no  means  thought  to  be  limited  to  the  statements 
with  which  his  name  will  be  found  to  be  connected ; 
indeed,  I  owe  him  more  than  I  can  well  express. 

I  have  been  careful  to  acknowledge  the  source 
whence  the  illustrations  are  taken,  and  I  have  to 
return  my  thanks  to  the  Publication  Committee  of 
the  Zoological  Society ;  to  Professor  Flower,  who 
only  added  one  more  to  a  number  of  acts  of  personal 
kindness  when  he  generously  put  at  my  disposal  all 
the  wood-blocks  which  were  in  his  own  possession  ; 
and  to  those  other  friends  who  have  allowed  me  to 
copy  figures  from  their  works. 

As  this  manual  is  written  on  lines  that  are  rarely 
followed,  I  shall  be  greatly  obliged  for  any  suggestions 
as  to  its  improvement,  or  for  corrections  of  any  errors 
which  may  have  found  their  way  into  it. 

F.    JEFFKEY   BELL. 

King's  College,  May,  1885. 


CONTENTS. 


CHAPTER  PAGE 

I.— INTRODUCTORY 1 

II.— AMCEBA 18 

III.-  THE  GENERAL  STRUCTURE  OF  ANIMALS  ...     23 

IV.— ORGANS  OF  DIGESTION 102 

V.— THE  BLOOD  AND  THE  BLOOD-VASCULAR  SYSTEM  .    181 

VI.— ORGANS  OF  RESPIRATION 210 

VII.— ORGANS  OF  NITROGENOUS  EXCRETION       .       .       .247 

VIII.— ORGANS  OF  SPECIAL  SECRETIONS         ....    265 

IX.— PROTECTING  AND  SUPPORTING  STRUCTURES     .       .    274 

X.— ORGANS  OF  MOVEMENT .370 

XL— VOCAL  ORGANS 387 

XII.— THE  NERVOUS  SYSTEM  AND  ORGANS  OF  SENSE      .    393 

XIII.— ORGANS  OF  REPRODUCTION 472 

XIV.— THE  DEVELOPMENT  OF  THE  METAZOA      .       .       .525 


COMPARATIVE  ANATOMY 
PHYSIOLOGY. 


CHAPTER     I. 

INTRODUCTORY. 

Comparative  anatomy  is  the  science  of  the 
structure  of  animals,  considered  in  their  relation  to 
one  another;  comparative  physiology  deals  with 
the  functions  of  the  parts  of  which  these  animals  are 
made  up,  and,  by  examining  different  forms  that 
present  various  kinds  of  activities,  it  throws  light  on 
the  essential  properties  of  living  matter. 

The  study  of  animals  is  but  a  part  of  the  wider 
science  of  the  study  of  organised  matter  generally, 
the  science  of  biology,  which  takes  plants  as  well  as 
animals  for  the  objects  of  its  investigations.  Under 
the  head  of  biological  studies  we  have,  therefore,  to 
group  (a)  those  which  regard  organisms  as  working 
machines,  capable  of  performing  various  functions ; 
these  studies  are  physiological,  whether  animals  or 
plants  be  separately  or  simultaneously  examined ; 
(b)  in  the  second  place,  the  parts  of  which  the  orga- 
nism is  made  up  may  be  investigated,  and  our  studies 
are  then  said  to  be  anatomical,  if  we  concern  our- 
selves with  isolated  types,  as  does  the  student  of 
human  anatomy  ;  or  they  are  morphological,  when  we 
compare  organisms  and  their  parts  one  with  another, 
and  try  to  draw  out  the  significance  of  isolated  facts, 
and  to  learn  their  bearing  on  the  general  scheme  of 
the  organisation  of  living  matter. 
B— 16 


2       COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  present  work  is  concerned  only  with  Animals ; 
but,  as  there  is  a  fundamental  resemblance  between 
Plants  and  Animals,  it  is  in  the  first  place  necessary 
to  enquire  into  the  characters  and  modes  of  activity 
of  living  matter,  pure  and  simple,  without  any  ques- 
tion as  to  whether  it  be  animal  or  vegetable. 

LIVING    MATTEE. 

Animals  and  plants  have  at  least  this  in  common, 
that  they  are  both  fashioned  out  of  a  material  which, 
in  all  its  essential  characters,  is  common  to  them 
both  ;  and,  whether  one  would  be  a  zoologist,  or  student 
of  animals,  or  a  botanist,  or  student  of  plants,  it  is, 
in  the  very  first  place,  necessary  that  he  should  have 
some  clear  and  exact  comprehension  of  what  are  the 
characters  and  what  are  the  modes  of  action  of  that 
primary  fashioning  substance  which  forms  the  material 
basis  of  living  creatures,  and  which  is  known  as 
protoplasm.  The  fact  that  the  sciences  of  zoology 
and  botany  have  to  do  with  this  " physical  basis"  of 
living  matter  separates  and  distinguishes  them  at  once 
from  such  studies  as  chemistry  or  physics,  with  which 
the  phenomena  of  life  have  no  necessary  connection. 

Living  is  distinguished  from  not- living:  matter 
by  several  important  and  easily  recognisable  charac- 
ters. It  would  seem  to  have  a  fundamental  and 
characteristic  composition ;  it  has  the  power  of  con- 
tinuing to  exist  by  taking  into  (nutrition),  and 
making  part  of  itself  (assimilation)  other  living  or 
even  not-living  matter.  Nutrition  and  assimilation 
lead  to  growth,  and  this  growth  is  succeeded  by  a 
stage  in  which  the  additional  material  obtained  is 
used  for  the  purposes  of  reproduction.  After  a 
time  a  living  organism  may  be  seen  to  be  unable  to 
withstand  the  action  of  the  surrounding  forces  in  the- 
midst  of  which  it  has  lived,  grown,  and  reproduced 
itself;  in  other  words,  its  activity  diminishes  and 


Chap,  i  ]     CHARACTERS  OF  LIVING  MATTER.  3 

diminishes,  until  at  last  it  dies.  From  this  dead 
matter,  living  material  can  never,  by  any  process  now- 
known  to  us,  be  produced  ;  for,  so  far  as  we  know, 
living  matter  can  only  proceed  from  other  living 
matter. 

As  the  chemist  is  only  able  to  acquire  definite  in- 
formation with  regard  to  the  chemical  composition  of 
living  matter  by  the  use  of  certain  treatments  which 
deprive  it  of  life,  we  cannot  speak  with  certainty  of 
more  than  the  broad  outlines  of  its  composition ;  but 
this,  at  least,  may  be  said  :  in  living  matter  (proto- 
plasm), the  four  chemical  elements,  oxygen,  hydro- 
gen, nitrogen,  and  carbon,  are  always  found,  and  with 
them  there  would  seem  also  to  be  associated  small 
quantities  of  sulphur  and  phosphorus.  It  is  possible, 
if  not  certain,  that  protoplasm  is  a  compound  of  a 
number  of  the  so-called  proteid  bodies,  and  it  is  quite 
certain  that  what  chemists  call  its  "  atomic  composi- 
tion "  is  very  high.  One  of  the  most  complex  bodies 
known  to  us  is  that  constituent  of  the  brain  which  is 
called  protagon ;  and  its  "  atomic  composition  "  has 
been  determined  to  be  C^B^NgPO^,  or  no  less  than 
509  atoms.  When  such  a  body  is  active,  fresh  chemi- 
cal changes  are  always  taking  place  within  it ;  it  is  in 
a  condition  of  unstable  equilibrium;  the  result  of 
such  change,  so  far  as  it  aftects  the  living  matter,  is 
loss  or  waste ;  in  addition  to  this,  living  matter  is 
always  taking  up  fresh  oxygen,  and  forming  carbonic 
acid,  of  which  it  has  to  free  itself.  These  activities 
combined  require,  as  may  be  supposed,  the  addition 
of  fresh  material  from  without;  that  is  to  say,  living 
matter  demands  food.  The  food  so  taken  in  may  or 
may  not  be  similar  in  composition  to  the  organism 
itself ;  but,  as  the  living  creature  has  wasted  through 
all  its  parts,  the  fresh  material  has  not  merely  to  be 
taken  in,  it  has  also  to  be  assimilated.  When  a  crystal, 
placed  in  a  solution  of  its  own  material,  grows,  it  does 


4      COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

so  by  merely  laying  on  the  fresh  molecules  outside 
those  already  formed  ;  protoplasm,  on  the  other  hand, 
makes  the  fresh  food,  which  may  or  may  not,  indeed 
need  not,  have  the  same  composition  as  itself,  an 
essential  part  and  parcel  of  itself. 

In  the  next  place  we  observe,  that  while  a  crystal 
under  the  conditions  just  now  mentioned  will  grow 
so  long  as  it  is  supplied  with  matter  of  similar 
chemical  constitution,  living  matter  only  grows 
when  assimilation  goes  on  at  a  quicker  rate  than 
destruction  or  waste.  Save  for  the  difficulties  of  ex- 
perimenting, there  is  no  reason  why  all  the  sulphate 
of  copper  in  the  world  should  not  (a)  be  brought  into 
one  huge  crystal,  and  (£)  so  remain.  It  is  not  so  with 
living  matter ;  for  every  organism  there  appears  to  be 
a  limit  of  growth,  and  when  that  is  reached,  all  the 
succeeding  matter  assimilated  goes  for  a  different  pur- 
pose. The  organism,  ceasing  to  grow,  begins  to  repro- 
duce its  kind,  and,  in  the  very  simplest  cases,  produces 
an  individual  exactly  similar  to  itself.  This  act  of 
reproduction  appears  to  be,  next  to  sustentation,  the 
primary  work  of  every  organism,  and  when  that  is 
completed,  we  often  observe  that  the  parent  organism 
begins  to  lose  its  activity  ;  it  becomes  the  prey  of  other 
living  organisms ;  or,  undergoing  gradual  decay,  the 
complex  mass  of  albuminous  matter,  which  we  call 
protoplasm,  and  associate  with  Hfe,  falls  away  into 
constituent  molecules  of  a  less  high  degree  of  chemical 
complexity. 

Assimilation,  growth,  reproduction,  death,  are, 
as  here  explained,  four  phases  in  the  history  of  living 
matter  which  at  once  and  sharply  distinguish  it  from 
crystalline  or  other  dead  material. 

Nor  is  this  all ;  if  we  set  one  crystal  against 
another  of  similar  composition,  or  if  we  try  to  rouse 
or  stimulate  a  crystal,  we  get  no  response.  With  living 
matter  the  case  is  very  different ;  roused  either  by 


chap,  i.]     CHARACTERS  OF  LIVING  MATTER.  5 

some  apparent  friend  or  enemy  in  the  water,  or  by  a 
touch  from  our  needle,  as  we  observe  it  under  the 
microscope,  a  mass  of  living  matter  will  be  found  to 
be  irritable.  In  consequence  of  this  irritability 

it  undergoes  some  change  converting  latent  into 
actual  energy,  and  this  is  most  frequently  and  most 
easily  seen  to  be  some  change  in  space,  or  in  the 
relations  of  its  parts ;  these  are  due  to  what  is  known 
as  the  contractility  of  living  matter.  In  other 
cases,  the  production  of  heat,  light,  or  electricity,  is 
the  expression  of  irritability. 

We  have  next  to  observe,  that  within  the  area  of 
any  given  mass  of  protoplasm,  there  may  be  move- 
ments of  its  parts ;  some  of  the  granules  seem  to 
stream  in  a-  more  or  less  regular  course  between 
those  on  either  side  of  them,  in  a  way  which  can  best 
be  understood  by  supposing  the  observer  to  be  raised 
above  and  to  be  able  to  note  the  movements  of  a  great 
crowd  of  passengers  in  a  busy  street ;  some  move 
faster  than  and  overtake  others,  some  collect  into 
more  or  less  small  crowds ;  others,  having  moved  on- 
ward for  a  certain  distance,  turn  aside  or  turn  back. 
This  streaming  movement  of  protoplasm  is  highly 
characteristic,  and  affords  a  proof  that  the  problem  of 
the  motile  activity  of  protoplasm  can  only  be  explained 
by  the  study  of  the  parts  of  which  it  is  made  up. 

Lastly,  thin  layers  of  non-granular  protoplasm 
are  sometimes  to  be  observed  gliding'  over  firm 
bodies  ;  by  these  means  the  whole  mass  is  enabled  to 
progress  in  a  forward  direction. 

The  study  of  streaming  movements  shov/s  us  that 
the  constituent  particles  do  not  move  around  any  fixed 
point,  but  freely  as  the  particles  of  a  fluid  substance. 
So  far  as  we  can  see,  these  movements  are  not  the 
result  of  any  external  cause ;  did  we  choose  to  allow 
that  a  simple  mass  of  protoplasm  had  a  "  will,"  we 
might  well  call  them  "  spontaneous"  or  "  voluntary;" 


6      COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

without  going  so  far,  we  must  allow  that  they  appear 
to  be  due  to  the  protoplasm  itself;  they  are  self- 
moved  or  automatic. 

Living  matter,  then,  is  irritable  and  automatic ; 
irritability  finds  expression  in  contractility,  or  in  the 
production  of  such  forces  as  heat,  light,  or  electricity. 

With  regard  to  its  general  physical  and  chemi- 
cal characters,  we  have  to  note  that  it  is  possessed  of 
great  cohesive  powers,  and  yet  is  very  extensile  ;  it 
does  not  mix  with  water,  but  it  swells  by  imbibition  ; 
it  may  expel  the  contained  fluid  in  the  form  of 
rounded  vacuoles,  and  bubbles  of  gas  are  sometimes 
apparent  in  it.  It  is  ordinarily  colourless,  and  re- 
fracts light  more  strongly  than  water ;  it  is  in  most, 
and  probably  in  all  cases,  slightly  alkaline  in  reac- 
tion. 

Before  we  leave  the  general  consideration  of  pro- 
toplasm, we  must  point  out  two  foreign  elements 
which  have  to  be  considered.  The  first  of  these  is  the 
presence  in  protoplasm,  as  we  ordinarily  observe  it,  of 
various  more  simple  chemical  compounds,  which  have 
the  form  of  granules  ;  these,  which  may  be  fatty  or 
starchy  bodies,  are  conveniently  grouped  together 
under  the  head  of  metaplasm;  they  may  be  re- 
garded as  owing  their  origin  to  the  changes  that  are 
constantly  taking  place  in  the  molecular  constitution  of 
the  protoplasm,  or,  in  other  word?,  as  waste  products 
not  yet  eliminated. 

The  second  is  a  general  motion  of  a  protoplasmic 
mass,  especially  when  of  particularly  small  size  (e.g. 
bacteria)  ;  this  movement  of  the  body  as  a  whole  is 
not  a  vital,  but  a  purely  physical  phenomenon,  as  may 
be  demonstrated  by  the  simple  experiment  of  rubbing 
up  a  little  gamboge  in  a  drop  of  water,  when  exactly 
the  same  movement  is  to  be  observed.  This  approxima- 
tion and  separation  of  small  particles  is  a  phenomenon 
which  has  attracted  the  attention  of  the  physicist,  by 


Chap,  i.]  THE  CELL.  7 

whom  it  must  be  explained ;  it  was,  however,  first 
observed  by  an  eminent  botanist,  and  is  consequently 
known  as  the  Browiiian  movement. 

The  term  cell  is  not  unfrequently  applied  to 
every  separate  mass  of  living  matter,  but,  in  conse- 
quence of  the  associations  connected  with  this  term, 
it  is  better  to  make  use  of  the  more  elaborate  though 
perhaps  more  intelligible  nomenclature  which  enables 
us  to  distinguish  between  the  different  characters  of 
"elementary  organisms."  When  attention  was  first 
directed  to  these  objects,  the  botanist  observed  that  in 
each  mass  of  protoplasm  there  was  a  portion  which, 
by  various  characters,  could  be  easily  distinguished 
from  the  rest,  and  which  might  be  very  appropriately 
spoken  of  as  the  nucleus ;  in  addition  to  this,  he 
saw  that  the  outer  portion  of  the  protoplasm  was  en- 
closed as  in  a  wall ;  he  spoke,  therefore,  of  the  whole 
as  a  cell,  with  a  cell  wall,  and  a  contained  nucleus. 

Later  on  it  was  found  that  the  protoplasm  (or 
"  sarcode,"  as  it  was  originally  called)  of  animals  was 
not  to  be  distinguished  from  that  of  plants,  and  it 
was  then  also  seen  that  it  was  only  in  very  rare  cases 
that  this  animal  protoplasm  was  enclosed  in  a  cell 
wall.  Thereby  the  very  first  conception  of  a  cell  was 
destroyed,  but  the  name  was  still  retained  as  a  con- 
venient term. 

Still  later  researches  revealed  the  at  first 
astonishing  fact  that  organisms  could  and  did  exist 
in  which  that  specially  modified  portion  of  the  proto- 
plasm which  had  been  called  the  "  nucleus "  was,  to 
all  appearance,  altogether  absent ;  some  naturalists, 
and  especially  some  physiologists,  now  regard  the 
nucleus  as  no  essential  part  of  the  cell.  On  the  other 
hand,  it  seems  better  to  recognise  in  our  nomenclature 
the  present  conditions  of  our  knowledge,  and  to  use 
for  the  "  elementary  organism "  some  other  definite 
term  than  that  around  which  so  many  battles  have 


8      COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

been  fought,  and  with  which,  perhaps,  no  few  super- 
stitions are  or  have  been  connected. 

We  will,  therefore,  follow  those  who  have  agreed 
to  the  suggestion  of  Prof.  Haeckel,  and  will  use  for 
the  elementary  organism,  whether  or  no  provided  with 
a  nucleus,  the  useful  and  suggestive  term  of  plastid. 
This  plastid,  or  unit  of  organic  structure,  is  com- 
posed of  protoplasm ;  it  may  be  without  a  nucleus, 
when  it  is  a  cytod  (or  cell-like  body),  or  it  may  have 
within  it  a  denser  mass,  which  is  very  feebly,  if  at  all, 
contractile,  the  nucleus;  in  which  case  it  is  a  cell. 
This  nucleus  is  ordinarily  provided  with  one  or  more 
smaller  nucleoli,  and,  possibly,  always  has  a  distinct 
investing  membrane.  It  would  appear  to  have  a 
special  chemical  composition,  inasmuch  as  while  a  cell 
when  treated  with  a  ten  per  cent,  salt  solution  leaves 
a  precipitate,  no  such  precipitate  is  stated  to  be  found 
when  a  cytod  is  subjected  to  the  same  reagent.  The 
body  so  precipitated  has  been  called  nuclein. 

Protoplasm,  then,  is  presented  to  us  in  the  form 
of  plastids,  and  these  plastids  may  either  be  without 
(cytods)  or  have  (cells)  distinct  nuclei.  All  organisms 
are  composed  of  one  or  more  cells,  or,  in  other  words, 
are  either  unicellular  or  multicellular.  The  former, 
as  much  as  the  latter,  are  capable  of  exhibiting  all 
the  .essential  phenomena  of  life. 

TISSUES    AND    ORGANS. 

"When  we  examine  the  different  stages  in  the 
history  of  a  developing  animal,  or  compare  a  series 
which  commences  with  low  and  passes  through  more 
highly  developed  forms,  we  find  a  gradual  increase  in 
the  complexity  of  the  parts ;  of  this  we  have  already 
had  an  example  in  comparing  the  cytod  with  the  cell, 
and  we  shall  observe  it  in  every  chapter  of  this  work. 
This  increase  in  complexity  is  termed  the  process  of 
differentiation. 


Chap.  I.]  TISSUES   AND    ORGANS.  9 

In  making  a  general  survey  of  animals  we  find 
that  the  lowest  consist  only  of  simple  cells  ;  later  on, 
the  cells  are  found  not  to  live  an  independent 
existence,  but  to  be  associated  one  with  another,  and 
different  groups  of  cells  are  seen  to  be  differentiated 
in  various  ways.  The  result  of  this  is  that  sets  of 
cells  come  to  have  different  characters  (some  are 
contractile,  others  irritable,  and  so  on),  and  these 
different  sets  are  what  are  known  as  tissues ; 
secondly,  we  observe  that  these  tissues  become 
connected  with  one  another  in  different  proportions 
and  relations,  so  as  to  give  rise  to  those  parts  of  the 
adult  which  take  on  particular  duties,  and  are  known 
as  organs. 

Looked  at  in  a  general  way,  and  without  taking 
any  notice  of  exceptional  cases,  we  observe  that 
there  are  tissues  in  an  animal  which  are  not  found  in 
a  plant ;  these,  which  are  distinguished  as  the  animal 
tissues,  are  such  as  have  a  relation  to  movement  or 
sensation  ;  in  other  words,  the  muscles  and  nerves  are 
animal  tissues.  On  the  other  hand,  plants  preserve, 
protect,  and  sustain  themselves,  and  the  corresponding 
tissues  in  animals  are  always  spoken  of  as  the 
vegetative ;  .of  these  we  may  find  convenient 
examples  in  that  outer  layer  of  the  body  which  is 
spoken  of  as  epithelium,  or  that  supporting  tissue 
which  is  known  as  bone. 

The  classification  of  organs  is  a  little  more  complex, 
but  it  will  be  convenient  to  give  it  now,  so  that  time 
and  space  may  be  saved  in  the  future. 

In  the  first  place  it  is  clear  that  the  vegetative 
functions  fall  under  three  great  heads ;  an  animal  has 
to  care  for  itself,  to  adapt  itself  to  or  move  through 
its  surroundings,  and  to  reproduce  its  kind.  And,  in 
the  second  place,  it  is  just  as  obvious  that  it  has  to 
perceive  what  is  going  on  around  it,  and  to  act 
accordingly.  We  have,  then  : 


io    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

(1)  Organs  of  internal  relations. 

i.   Protective.— Examples :  Skin,  shell. 

ii.  Nutritive. — Examples :  Digestive  tract  (nu- 
trient) ;  heart  and  blood-vessels  (circulatory). 

iii.  Purifying1.— Gills,  lungs  (carbonic  acid) ;  kid- 
neys (nitrogenous  products). 

(2)  Organs  of  external  relations. 

iv.  LiOCOmotor. — Limbs,    etc.      (compounded    of 

skeletal  and  muscular  tissues). 
v.  Prehensile. — Limbs,  etc.  (compounded  of 

skeletal  and  muscular  tissues). 
vi.  Offensive.— Teeth,  claws,  electrical,  odorous 

organs. 

(3)  Reproductive. 

(a)  Germ -producing  glands  :  testes,  ovaries,  which  are 

essential. 
(/3)  Copulatory :  penes,  etc.,  which  are  accessory. 

(4)  Sensory. 

(a)  Organs   which   receive  impressions  ;  eye,  ear, 

etc.,  brain. 
(j8)  Organs  which  Stimulate  other  organs  ;   brain. 


METHODS    OF    COMPARISON. 

When  an  anatomist  has  acquired  a  positive 
knowledge  of  a  certain  number  of  selected  forms  of 
life,  he  proceeds  to  convert  his  empirical  acquaintance 
with  facts  into  science  by  reasoning  upon  the  infor- 
mation which  he  has  acquired.  In  this  operation  he 
makes  great  use  of  the  fertile  method  of  comparison, 
remembering  the  words  of  Buflbn,  "  Ce  riest  qu'en 
comparant  que  nous  pouvons  juger."  Like  things, 
however,  must  be  compared  with  like,  or  confusion 
will  inevitably  result.  We  must,  therefore,  lay  down 
certain  rules  to  guide  us  in  these  kinds  of  enquiries, 
for,  though  no  one  would  attempt  to  compare  a  heart 
with  a  lung,  many  would,  at  first,  more  willingly 
compare  the  leg  of  a  man  with  that  of  a  cockroach, 
than  the  fin  of  a  perch  with  the  wing  of  the  sparrow  ; 
yet  the  latter  is  the  more  justifiable  proceeding. 


Chap.  I.]  HOMOLOGY.  1 1 

The  reason  for  this  is  plain,  the  moment  we  clearly 
understand  what  object  the  comparative  anatomist 
has  before  him ;  it  is  that  of  coming  to  some  general 
conclusions  as  to  identity  or  community  of  structure  ; 
for  this  purpose,  then,  he  is  not  to  compare  parts 
that  have  the  same  function,  but  those  that  are  formed 
in  the  same  kind  of  way.  The  physiologist,  on  the 
other  hand,  looking  at  organs  as  parts  of  a  machine, 
examines  together  those  that  do  the  same  thing. 
When  we  compare  parts  morphologically,  we  must  not 
be  content  merely  with  an  analogy  between  them, 
we  must  be  careful  that  there  is  a  homology  or 
real  resemblance. 

The  first  criterion  of  homological  parts  is  their 
development  from  similar  embryonic  structures  ;  such 
are  the  wing  of  a  bird  and  the  leg  of  a  horse.  But 
a  further  question  now  arises  ;  why  have  these  wings 
and  legs,  which  in  their  completed  condition  are  so 
different  from  one  another,  a  similar  structure  in  the 
embryo  1  The  answer  to  this  is  given  by  the  doctrine 
of  descent,  which  supposes  that  the  bird  and  the 
horse  had  in  the  past  a  common  ancestor,  provided 
with  limbs  simpler  in  structure  than  those  of  either 
bird  or  horse,  but  having  essentially  that  which  they 
have  now,  or  which  they  have  had,  and  from  which 
they  are  both  derived ;  a  true  and  complete  homology 
of  parts  is,  then,  only  to  be  found  between  animals 
which  have  had  a  common  ancestor  provided  with  the 
part  to  be  compared.  This  complete  homology  may 
be  conveniently  spoken  of  as  homogeny  (Ray 
Lankester). 

It  is  very  necessary  to  have  before  the  mind  this 
idea  of  community  of  descent,  because  we  shall  con- 
stantly meet  with  cases  in  which,  with  a  very  close  re- 
semblance in  structure  and  mode  of  development,  there 
is  not  a  complete  identity  in  descent.  For  example, 
all  mammals  and  all  birds  are  provided  with  four 


12    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

cavities  in  their  hearts  :  two  auricles  and  two  ven- 
tricles ;  but  it  is  certain  that,  whatever  was  the  animal 
that  was  the  nearest  common  ancestor  to  the  two,  it 
had  only  one  ventricle.  The  right  and  left  ventricles 
of  the  hearts  of  birds  and  mammals  are,  then,  not 
homologous  in  the  sense  of  being  homogenetic  ;  they 
have  been  acquired  independently  by  the  two  groups, 
in  consequence  of  certain  physiological  needs ;  they 
are  the  result  of  similar  modifying  forces,  and  are 
homoplastic,  but  not  homogenetic  parts. 

DEVELOPMENT. 

The  last  point  to  which  the  student  must  be  intro- 
duced is  one  of  the  very  greatest  importance.  If  we 
study  the  animal  kingdom  throughout,  we  find  that, 
starting  from  the  simplest  mass  of  protoplasm, 
we  are  gradually  led  to  the  complex  and  elabo- 
rate structural  and  functional  arrangements  which 
are  found  in  so  highly  organised  an  animal  as 
man  himself.  If,  on  the  other  hand,  we  study  the 
developmental  history  of  a  highly  organised  form, 
we  find  that  it  starts  from  a  simple  mass  of  pro- 
toplasm, the  egg,  or  ovum,  as  this  plastid  is  called ; 
this  cell  gradually  becomes  more  and  more  elaborated, 
and  takes  on  the  more  complex  arrangement  which 
may  be  seen  in  its  parent ;  we  observe,  that  is,  that  not 
only  are  there  a  number  of  stages  in  the  different  re- 
presentatives of  the  animal  world,  but  that  there  are 
also  a  number  of  stages  in  the  structural  history  of 
every  individual ;  and  we  may  go  yet  a  step  farther, 
and  say  that  in  a  broad  and  general  way  there  is  a 
complete  parallelism  between  the  two. 

The  results  of  the  investigations  and  considerations 
which  flow  from  a  study  of  the  facts  here  indicated 
are  best  expressed  in  an  aphorism,  which  may  at  once 
be  laid  to  heart,  and  which  will  be  abundantly  proved 
by  a  study  of  development  and  comparative  anatomy  : 


Chap,  i.]  EVOLUTION.  13 

"  The  history  of  the  individual  is  a  compressed  epitome 
of  the  history  of  the  race." 

Those,  therefore,  who  desire  to  obtain  a  complete 
knowledge  of  animals  or,  indeed,  of  any  one  animal, 
must  not  be  contented  with  an  account  of  the  anatomy 
of  the  adult ;  they  must  direct  their  attention  also 
to  its  development,  and  become  the  students  of 
Embryology,  while  they  must  no  less  take  care  to 
study  the  history  of  the  animal,  or  of  its  allies,  in  the 
past  ages  of  the  world,  or  to  know  something  of  its 
Palaeontology. 

These  two  branches,  Embryology  and  Palseont- 
ology,  are  of  the  greatest  assistance  in  an  endeavour 
to  obtain  some  clear  idea  of  the  morphology  of  animals  ; 
but  a  weapon  no  less  sure  and  no  less  important  is  that 
of  comparison,  by  means  of  which  similar  parts  in 
different  organisms  are  studied  and  explained ;  no 
better  aid  to  safe  judgment  can  be  afforded,  and  it  must 
be  used  unceasingly  and  unsparingly. 

EVOLUTION. 

The  great  maze  and  mass  of  facts  which  are  found 
in  works  on  zoology  or  comparative  anatomy  are 
hardly  to  be  held  together  without  the  bond  of  phi- 
losophy ;  the  grouping  of  facts,  and,  still  more,  the 
grouping  of  animals,  must  be  always  more  or  less  un- 
intelligent, mechanical,  and  artificial,  unless  we  make 
some  use  of  some  kind  of  explanation.  That  which 
we  shall  use  here  will  be  founded  on  the  belief  that 
there  is  a  blood  relationship,  or  relationship  by  descent 
and  inheritance,  between  every  member  of  the  animal 
kingdom;  and  that,  were  it  possible  to  know  all  the 
facts,  we  could  make  a  genealogical  tree  for  animals, 
which  should  be  as  exact  and  definite  as  the  family 
tree  which  is  drawn  up  by  the  genealogist  or  the 
herald.  That  division  of  biology  which  busies  itself 
with  genealogical  problems  is  known  as  Phylogeny. 


14    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

All  the  steps  in  the  differentiation  and  elaboration 
of  organised  beings  are  examples  of  that  process  of 
evolution,  which,  when  based  on  a  belief  in  the 
existence  of  a  blood  relationship  between  animals,  is 
known  as  the  doctrine  of  descent.  In  an  attempt  to 
understand  how  this  has  worked,  we  make  reference  to 
two  different  series  of  facts.  We  have,  in  the  first 
place,  to  make  certain  generalisations  as  to  the  way  in 
which  the  differences  have  been  brought  about,  and  we 
have,  in  the  second,  to  consider  what  are  the  essential 
properties  of  living  matter  which  may  be  regarded  as 
the  determining  factors  in  the  evolution  of  organised 
material 

The  generalisations  made  from  a  number  of  obj 
served  facts  may,  if  this  definition  be  borne  in  mind, 
be  called  the  laws  of  evolution;  they  have  been  thus 
enunciated  by  Professor  Huxley  : 

(1)  There  has  been  an  excess  of  development  of 
some  parts  in  relation  to  others. 

(2)  Certain   parts    have    undergone   complete   or 
partial  suppression. 

(3)  Certain  parts,  which  were  originally  distinct, 
have  coalesced. 

Let  us  apply  these  laws  to  a  concrete  example,  and 
select  for  study  the  fore-foot  of  a  camel  In  the  more 
primitive  mammalia  there  were  five  fingers  or  digits, 
each  connected  by  a  metacaqDal  or  palm  bone  with 
the  wrist,  and  these  five  sets  of  digits  and  metacarpals 
were  of  subequal  size.  In  the  hoofed  group  of  animals 
the  first  of  these,  or  thumb,  disappeared,  as  in  the  case  of 
the  modern  pig ;  the  two  that  were  now  outermost,  the 
second  and  fifth,  became  smaller  and  smaller,  as  in  the 
sheep  or  deer,  and  finally,  as  in  the  camel,  disappeared 
altogether.  Here  we  have  various  stages  of  law  2. 
This  loss  of  the  outer  was  accompanied  by  an  increase 
in  the  size  of  the  median  digits  and  metacarpals 
(law  1),  and  in  the  more  or  less  complete  fusion  of 


chap,  i.i      HEREDITY  AND   VARIABILITY.  15 

the  third  and  fourth  metacarpals  one  with  the  other 
(law  3) ;  the  result  of  this  last  process  being  the 
formation  of  a  bone  which  at  its  lower  end  only 
gives  any  obvious  indication  of  its  primitively  double 
nature. 

The  characteristics  of  protoplasm  which  appear  to 
be  the  determining  factors  of  evolution,  are  (1)  its 
power  of  producing  an  organism  like  to  itself;  and  (2) 
the  fact  that  no  child  or  parent,  or  any  two  children, 
are  exactly  similar  one  to  another.  The  first  of  these 
principles  is  known  as  that  of  heredity,  the  second 
as  that  of  variability.  It  is  obvious  that  the 
second  principle  only  comes  into  action  because  of  the 
differences  in  the  surroundings  of  every  individual 
plastid  ;  the  greater  the  homogeneity  of  the  surround- 
ings, the  greater  the  likenesses  between  the  plastids. 
The  law  of  heredity  may  consequently  be  compared  to 
the  first  law  of  motion  (Gasquet). 

Organisms,  therefore,  tend  to  resemble  their 
parents,  but,  being  more  or  less  differently  affected  by 
surrounding  media  and  objects,  diverge  more  or  less 
from  the  parent  stock  ;  the  greater  the  differences 
in  environment,  the  greater  the  differences  between 
parent  and  child.  This  is  a  fact  so  well  known  to 
us  all  that  we  need  not  enlarge  upon  it  here. 

ANIMALS    AND    PLANTS. 

While  the  conviction  that  there  is  an  essential 
unity  between  animals  and  plants  may  be  taken  as 
one  of  the  most  important  results  of  modern  biology, 
we  have  to  note  that  along  the  two  lines  of  organisa- 
tion the  constituent  protoplasm  has,  on  the  whole,  de- 
veloped special  characteristics.  In  other  words,  we  are 
not  able  always  to  say  definitely  whether  a  given  uni- 
cellular- organism  is  an  animal  or  a  plant ;  but  we  can 
always  with  certainty  point  to  the  differences  which 
distinguish  a  rose  from  a  bee. 


1 6    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Thus  the  form  (1)  of  a  plant  is  diffuse  and  arbores- 
cent, that  of  an  animal  oblong  and  rounded.  A  plant 
lives  on  (2)  carbonic  acid  and  mineral  salts,  but  an 
animal  requires  albuminoid  foods.  These  foods  are  in 
the  plant  taken  in  (3)  by  the  porous  tissues,  and  there 
is  no  distinct  mouth  as  there  is  in  all  but  the  lowest, 
and  in  the  majority  of  parasitic  animals.  The  secre- 
tions (4)  of  a  plant  are  non-nitrogenous,  while  some  of 
the  waste  products  of  an  animal  always  contain 
nitrogen.  In  their  habits  (5)  we  find  that  plants  are 
fixed,  and  animals  locomotive.  And,  lastly,  (6)  as  to  the 
characters  of  their  cells,  we  find  that  plants  have  a  cell 
wall  formed  of  that  ternary  compound  which  is  known 
as  cellulose,  while  the  wall  of  an  animal  cell,  when 
present,  is  derived  directly  from  the  cell  protoplasm. 

To  nearly  all  the  statements  now  made  an  ex- 
ception may  be  found  :  thus  (1)  cacti  and  fungi  are 
certainly  not  arborescent  or  diffuse,  while  polyps  as  cer- 
tainly are.  (2)  Fungi  appear  to  require  some  more 
complex  compound  than  merely  carbonic  acid  and 
mineral  salts,  but  such  a  body  as  ammonium  tart  rate 
will  give  the  nourishment  required ;  every  animal 
known  to  us  requires  albuminoid  food,  and  dies  when 
deprived  of  it.  (4)  It  is  quite  true  that  plants  do 
not  give  off  nitrogenous  excreta  ;  but  their  protoplasm, 
it  must  always  be  remembered,  is  capable  of  forming 
them  ;  on  the  other  hand,  all  the  excreta  of  an  animal 
are  not  nitrogenous;  Ascidians  (and,  if  they  are 
truly  animals,  some  of  the  Cilio-flagellata)  form  cellu- 
lose. The  latter  and  some  low  worms  have  been  ob- 
served to  form  starch,  and  sugar  is  a  ternary  compound 
formed  by  various  animals.  The  well-known  Yolvox 
offers  (5)  an  exception  to  the  statement  that  plants  are 
fixed,  and  polyps  and,  to  a  large  extent,  stalked  Echi- 
noderms,  to  the  statement  that  animals  are  locomotive. 
Lastly,  some  of  the  lowest  plants,  such  as  Myxomycetes, 
have  their  protoplasm  naked,  while  the  just-mentioned 


chap,  i.]  ANIMALS  AND  PLANTS.  17 

Cilio-flagellata  have  cellulose  in  their  cell-walls,  and 
the  so-called  matrix  of  cartilage  cells  does  not  appear 
to  be  directly  formed  from  the  cells  themselves. 

This  enumeration  of  differences  or  resemblances  is, 
after  all,  unsatisfactory,  and  will,  with  the  progress  of 
knowledge,  come,  no  doubt,  to  be  regarded  as  mis- 
leading ;  for  the  present,  it  will  not  fail  in  its  object 
of  impressing  on  the  student  the  broad  and  general 
characteristics  of  animals  and  plants  as  we  now  know 
them  ;  but  there  must  be  added  to  it  a  reminder  that 
among  the  higher  members  of  the  Droseracese  we  find 
plants  (a)  whose  leaves  have  in  some  forms  the  power 
of  movement  when  excited ;  (#)  the  glands  of  their 
leaves  are  able  both  to  digest  and  to  absorb  animal 
matters ;  and  (7)  the  normal  electrical  current  is, 
when  these  leaves  are  irritated,  disturbed  in  the  same 
manner  as  is  that  of  a  contracting  animal  muscle. 

The  general  relation  of  animals  to  plants  is  well 
shown  in  the  following  table  (Brass) : 

^Plants- 


use  up  form 

carbonic  acid,  water,  nitrates,         oxygen,  carbohydrates,  fat,  albumen, 


form  use  up 


The  fact  that,  in  sunlight,  green  plants  (that  is, 
plants  containing  chlorophyll)  give  off  oxygen  has  led 
some  to  think  that  plants  take  in  carbonic  acid  and 
exhale  oxygen  ;  but  plants  as  much  as  animals  give  off 
carbonic  acid  as  a  waste  product.  If  or  when  an  ani- 
mal contains  chlorophyll  grains,  it  as  much  as  a  plant 
will  give  off  oxvgen  under  the  influence  of  sunlight. 
c— 16  ' 


18 


CHAPTER  IT. 

AMCEBA. 

IT  has  been  wisely  said  that  "the  highest  laws  of  our 
science  are  expressed  in  the  simplest  terms  in  the 
lives  of  the  lowest  orders  of  creation  "  (Paget) ;  and 
it  will  be  well,  therefore,  to  commence  our  studies 
with  a  close  investigation  into  the  characters  of  one  of 
the  simplest  of  living  animals. 

The  word  Amoeba  is  a  generic  term,*  which  is 
applied  to  a  number  of  forms,  which  have  in  common 
the  following  characters;  they  are  more  or  less 
minute  specks  of  nucleated  protoplasm,  without  any 
wall  or  membrane  limiting  their  surface,  and  they  are 
capable  of  pushing  out  processes  of  their  body  sub- 
stance from  any  part  or  point  of  it.  They  are  some- 
times as  much  as  one-hundredth  of  an  inch  in 
diameter,  but  they  in  all  cases  require  the  assistance 
of  a  microscope  of  high  powers  for  their  satisfactory 
study. 

If  we  place  one  on  a  glass  slide,  and,  after  allowing 
it  to  become  used  to  its  new  position,  examine  it 
under  the  microscope,  we  shall  at  once  see  how  ap- 
propriate is  the  name  that  has  been  given  it.  Its 
form  is  never  constant  for  more  than  a  few  moments 
together,  as  we  can  best  demonstrate  by  making  a 
sketch  of  its  shape  once  every  minute  for  some  five  or 
six  times. 

These  changes  in  form  are,  we  know,  expressions 
of  the  irritability  and  contractility  of  the  protoplasm. 

*  The  possibility  that  a  number  of  so-called  Amoebae  are 
stages  in  the  life-history  of  animals  or  plants  does  not  affect  the 
question  here  dealt  with. 


Chap.  II.] 


AMCEBA. 


T9 


Looked  at  more  closely,  we  see  in  it  evidences  of  dif- 
ferentiation of  structure;  the  mass,  small  as  it  is, 
is  not  homogeneous ;  the  outer  portion  is  denser  and 


Fig.   1.— Amoeba. 
n,  Nucleus  ;  cv,  contractile  vacuoles. 

clearer  (Fig.  1)  than  the  inner,  which  is  more  fluid 
and  granular.  Although  these  two  portions  are  not 
sharply  marked  off  from  one  another,  it  is  convenient 
to  have  definite  names  by  which  to  distinguish  them, 
and  we  will  speak  therefore  of  an  ectosarc,  and  an 
endosarc.  Within  the  endosarc  we  see  a  disk- 
shaped  or  rounded  body  which  retains  its  form,  while 


20    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  protoplasm  around  it  is  changing ;  this  is  the 
nucleus  (n),  and  within  it  is  a  smaller  body,  the 
little  nucleus,  or  niicleolus.  In  the  ectosarc  we 
have  to  observe  a  space  which  opens  slowly,  and  con- 
tracts rapidly ;  its  power  of  contraction  may  be  seen 
to  be  independent  of  that  of  the  general  mass  of  proto- 
plasm. This  space  (the  contractile  vacuole,  cv) 
appears,  though  we  cannot  speak  with  certainty,  to 
be  a  kind  of  pump,  whereby  water  is  taken  into  and 
forced  out  of  the  body  ;  the  water  that  enters  must 
bring  with  it  a  certain  quantity  of  oxygen,  which  is 
a  prime  necessity  of  every  living  organism,  whether 
it  be  plant  or  animal ;  while  the  water  that  is  forced 
out  of  the  body  must  carry  with  it  a  certain  quantity 
of  those  waste  products  which  always  appear  when  a 
living  body  is  in  active  function. 

The  contractile  vacuole,  then,  would  appear  to 
effect  for  the  amoeba  the  two  processes  of  respiration 
and  of  purification,  which,  in  higher  animals,  are  per- 
formed by  definite  organs. 

It  will  at  once  be  noticed  that  there  is  no  special 
point  by  which  food  enters,  or  what  is  useless  in  that 
food  escapes  from  the  amoeba ;  in  other  words,  there 
is  neither  mouth  nor  anus.  But  it  will  almost  as 
soon  be  seen  that  this  naked  cell  has  no  need  of 
either  the  one  or  the  other ;  it  flows  around  the  food 
it  needs,  and  it  flows  away  from  the  waste  or  useless 
matter  which  is  of  no  further  use  to  it. 

Just  as  there  is  no  special  inlet  for  the  food,  so 
there  is  no  part  of  the  cell  which  can  be  said  to  be  es- 
pecially digestive  in  function.  We  can  best  see  what 
happens  to  the  food  when  it  is  a  green-coloured  plant ; 
when  such  is  under  observation  we  find  that  it 
gradually  breaks  up  within  the  amoeba,  that  it 
gradually  loses  its  green  colour,  and  finally  disappears  ; 
if  it  be  a  diatom  that  has  been  flowed  around,  we 
may  observe  in  time  that  the  undigested  case  will  be 


chap.  ii. j  AMCEBA.  21 

left  behind.  The  cell,  then,  of  which  the  amoeba 
consists,  is  capable  of  taking  in  food,  and  of  making 
it  part  of  itself ;  it  can,  in  fine,  effect  all  the  opera- 
tions of  nutrition. 

The  flowing  around  food  is  only  an  expression  of 
that  general  locomotor  activity  of  the  amaba  which 
finds  a  more  general  expression  in  those  remarkable 
changes  in  form  to  which  we  have  already  directed 
attention.  These,  when  studied  in  detail,  are  found 
to  be  effected  in  the  following  fashion.  At  some 
point  of  the  body  where  the  contour  is  smooth  and 
rounded  a  little  knob  of  ectosarc  may  be  seen  to  be 
protruded,  and  to  widen  out  as  it  increases  in  size ; 
the  cavity  in  its  interior  which  is  thus  formed  becomes 
filled  with  endosarc  which  flows  into  it.  The  pro- 
trusion is  at  first  broad  or  lobate,  and  it  may  so 
remain  ;  or  it  may  increase  in  length  and  diminish 
in  proportionate  breadth,  or  it  may  even  become 
branched  at  its  free  extremity.  Such  an  out-pushing 
of  the  substance  of  the  naked  cell  is  spoken  of  as  a 
pseudopodium  (false  foot).  When,  as  often 
happens,  several  small  pseudopodia,  or  one  or  a  few 
of  large  size  are  given  off  close  to  one  another,  and  if 
the  pseudopodia  are  not  at  the  same  time  protruded 
from  the  opposite  surface  of  the  cell,  then  the  whole 
mass  follows  the  pseudopodia,  and  there  is  a  general 
movement  of  the  amoeba ;  at  such  a  time  we  can 
distinguish  an  anterior  from  a  posterior  end. 

The  amoeba,  then,  feeds,  grows,  and  moves  about, 
takes  in  oxygenated  water,  and  gets  rid  of  waste 
material;  exhibits,  in  fine,  all  the  essential  pheno- 
mena of  internal  and  external  relation  ;  it  does  not 
exhibit  anything  more  than  a  general  irritability,  but 
as  it  does  answer  to  stimuli  from  without,  it  presents 
us  with  a  copy,  as  it  were,  of  the  changes  that  occur 
in  ourselves  when  we  are  acted  on  by  external  stimuli. 
It  performs  all  the  actions  that  are  essential  to  our 


22    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

idea  of  an  individual  living  for  itself.  But  it  does 
more  than  this  ;  it  performs  also  the  function  that  is 
necessary  for  the  continuance  of  the  species  of  which 
it  is  a  representative.  It  reproduces  itself. 

In  the  simplest  case  the  act  of  reproduction  is 
effected  thus;  the  nucleus  elongates,  becomes  con- 
stricted in  its  middle,  and  divides  into  two.  As  this 
division  is  being  effected  the  surrounding  protoplasm 
becomes  divided  into  two  masses,  each  of  which 
accompanies  one  half  of  the  nucleus.  As  a  result  of 
this  process  we  have  two  individuals  where  before 
we  had  one,  and  they  differ  only  from  the  amoaba 
which  we  have  been  previously  studying  by  their 
smaller  size ;  as  our  first  amoeba  has  altogether  dis- 
appeared, it  is,  to  all  practical  purposes,  dead ;  and 
we  have,  then,  in  this,  the  simplest  condition  of 
reproduction,  the  death  of  the  parent  absolutely  co- 
temporaneous  with  the  appearance  of  a  new  generation. 
This  process  of  reproduction  is  that  which  is  known 
as  fission. 

Another  method  is  also  observed  in  the  amceba, 
which  may  be  regarded  as  a  modification  of  that  of 
fission.  A  small  portion  (bud)  of  non-nucleated 
protoplasm  is  gradually  separated  off  from  the  rest  of 
the  mass ;  this  increases  in  size,  and  develops  within 
itself  a  new  nucleus,  so  that  it  becomes  exactly 
similar  to  its  parent,  which,  in  this  case,  continues  to 
exist.  Here  we  have  reproduction  effected  by  bud- 
ding, or  gem  million. 

Notwithstanding  all  the  functions  performed  by 
this  minute  mass  of  protoplasm,  it  will  be  observed 
that  there  is  nothing  in  the  cell  to  which  we  could 
correctly  give  the  name  of  an  organ.  We  are  in  the 
presence  of  life,  but  hardly  of  organisation. 


CHAPTEK  IIL 

THE  GENERAL  STRUCTURE  OF  ANIMALS. 

BEFORE  proceeding  to  a  comparative  account  of  the 
structure  and  functions  of  the  organs  of  different 
animals,  it  will  be  necessary  to  introduce  the  student 
to  the  broader  characteristics  of  the  groups  into  which 
the  animal  kingdom  has  been  divided.  What  fol- 
lows in  this  chapter  is  to  be  regarded  as  having  that 
aim  alone  ;  it  is  in  no  way  to  be  looked  upon  either 
as  a  classification  of  animals,  or  even  as  an  intro- 
duction to  it,  and  it  is  to  be  used  rather  as  a  kind  of 
guide  to  the  relative  position  of  any  animal  that  may 
be  mentioned  in  the  succeeding  chapters.  So  far  as  is 
possible  in  the  necessities  of  the  case,  it  has  been  so 
prepared  as  to  hinder  rather  than  to  aid  the  student 
in  any  attempt  to  commit  to  memory  a  system  of 
classification ;  for  it  is  certain  that  there  is  nothing  less 
fruitful  in  good  result  than  a  parrot-like  acquaintance 
with  what  is  only  a  compressed  epitome  of  the  more 
certain  results  of  zoological  enquiries,  but  which,  it  is 
to  be  remembered,  may  at  any  time  be  profoundly 
modified  by  further  investigation.  What  is  called  a 
classification  of  the  animal  kingdom  is  nothing  more 
or  less  than  a  precis  of  our  knowledge  at  a  given  mo- 
ment, and,  at  its  best,  can  never  be  more  than  rela- 
tively correct. 

On  the  other  hand,  the  sketch  that  follows  may  be 
of  use  as  indicating  the  general  course  of  development, 
taken  along  different  lines  by  different  kinds  of 
animals. 

The    simplest    animals    essentially    resemble    an 


24    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Amoeba  in  this  particular,  that,  for  the  whole  period 
of  their  lives,  all  the  functions  of  the  organism  are 
performed  by  a  single  cell ;  and,  even  where  cells 
remain  collected  into  a  colony,  each  individual  member 
of  that  colony  performs  all  its  own  duties,  and  affords 
no  assistance  to  the  rest ;  there  is  no  division  of 
labour. 

In  the  higher  animals  a  very  different  phenomenon 
is  seen;  here  again  the  whole  organism  is,  indeed, 
composed  of  cells  or  cell-derivates  ;  but,  howsoever  com- 
plex it  may  become,  it  starts  always  on  the  cycle  of 
its  existence  under  the  form  of  a  single  cell.  This  cell, 
which  is  known  as  the  ovum  or  egg-cell,  undergoes  a 
series  of  divisions  by  means  of  which,  two,  four,  eight 
....  cells  are  produced,  and  these  become  arranged  in 
definite  fashion,  and  take  on  more  or  less  well-defined 
functions.  Here,  then,  different  parts  of  the  organism 
have  different  duties,  or,  in  other  words,  there  is 
division  of  labour. 

The  first  or  lower  group  of  organisms  are  asso- 
ciated together  as  the  Protozoa;  the  second,  or 
those  that  come  after  them,  form  the  division  of  the 
JVIetazoa.  Did  we  desire  to  use  less  objective  terms, 
we  might  adopt  for  these  groups  the  corresponding 
terms  of  Cytozoa  and  Histozoa  (Maupas),  which 
conveniently  direct  attention  to  the  essential  differ- 
ence in  the  cells  of  the  protozoan,  and  the  tissues 
of  the  metazoan  organism. 

In  attempting  to  arrange  either  of  these  divisions, 
we  are  met  at  once  by  the  fact  that  the  changes  which 
have  taken  place  in  organisms  have  been  in  two  lines 
or  directions ;  there  has  been  progress,  and  there 
has  been  degeneration.  The  former  we  shall  find 
to  be  more  intimately  associated  with  a  free  and  active 
life,  and  a  ready  power  of  adaptation  to  changed  cir- 
cumstances ;  the  latter  to  a  fixed  and  often  to  a  para- 
sitic mode  of  existence. 


Chap,  in.]  GROUPS  OF  PROTOZOA.  25 


I.    PROTOZOA. 

For  our  purposes  we  shall  find  it  convenient  to 
divide  the  Protozoa  into  three  great  groups,  one  of 
which  has  become  degraded  by  parasitism  ;  these  are 
the  Sporozoa,  of  which  the  best  known  division  are 
the  Gregarinida ;  the  others,  one  of  which  is  dis- 
tinctly higher  than  the  other  group,  may  be  called  the 
Sarcodina  and  the  Infusoria. 

Of  the  Sarcodina,  the  best  type  is  the  common 
Amceba,  which  we  have  already  studied ;  like  it,  all 
the  members  of  the  group  move  about  and  take  in 
their  food  by  means  of  those  movements  of  the  proto- 
plasm of  the  cell  which  result  in  the  formation  of 
pseudopodia,  and  they  reproduce  themselves  either  by 
division  or  by  budding. 

In  the  Infusoria  the  amcBbiform  character  is 
lost,  and  the  cell  has  and  retains  a  definite  form  ;  the 
ectosarc  ordinarily  sheds  out  a  structureless  mem- 
brane. This  encloses  the  softer  protoplasm  which 
makes  up  the  rest  of  the  organism,  giving  oft' 
delicate  processes  which  make  their  way  through 
the  limiting  membrane :  these  processes,  or  cilia, 
are  typically  developed,  are  portions  of  proto- 
plasm which  retain  their  contractile  power,  and  form 
the  chief  means  of  progression.  Owing  to  the  pre- 
sence of  the  covering  membrane  or  cuticle^  it  is  neces- 
sary that  there  should  be  at  some  point  an  opening  in 
the  cell  (cytostome),  by  means  of  which  food  may, 
at  any  rate,  enter ;  this  opening  is  ordinarily  spoken 
of  as  the  mouth  ;  in  addition  to  it  there  is  sometimes  a 
second  orifice  developed,  which  has  the  function  of  an 
anus  (cytoproct). 

The  third  division  of  the  Protozoa  are  the  de- 
graded parasitic  forms,  of  which  the  Gregarine  is 
an  excellent  example.  Though  these  cells  are  covered 
in  by  a  distinct  membrane,  there  is  no  orifice  or 


26    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

mouth  by  which  the  food  can  enter ;  living  as  they  do 
in  the  digestive  tract  or  other  cavities  of  the  bodies  of 
higher  animals  in  which  nutritious  matter  is  abundant, 
they  obtain  such  food  as  they  require  by  the  mere 


JFig.  2  A.— Gromia,  showing  the  teat  and  the  protruding 
protoplasm. 

physical  process  of  osmosis.  Similarly,  having  ceased 
to  lead  a  free  life,  and  abiding  now  in  closed  spaces, 
they  have  lost  the  cilia  which  were  possessed  by  the 
infusorian  and  exhibit  instead  a  slow  serpentine 
movement  which  is  effected  by  the  ectosarc. 

The  Sarcodina  are  conveniently  divided  into  three 
great  divisions  : 


Chap.  III.] 


GROUPS  OF  PROTOZOA. 


27 


I.  Rhizopoda;  example:  Amoeba,  Gromia,  Nummu- 

lites. 

II.  Heliozoa  ;    example :    Actinophrys    (Sun    animal- 
cule). 
III.  Radiolaria ;  example  :  Acanthometra,  Chilomma. 


Fig.  2  B.— Actinophrys  sol,  showing  the  yacuolated  ectosarc,  the  finely 
granulated  endosarc,  the  nucleus,  contractile  vacuole.  and  pseu- 
dopodial  filaments.  (After  Leidy.) 


Leaving  out  of  our  consideration  those  simple  and 
incompletely  known  forms  in  which  no  nucleus  is 
developed  in  the  protoplasm  (Wlonera),*  we  may  dis- 
tinguish the  naked  Amoeba-like  Rhizopoda  from  those 

*  It  is  possible  that  in  such  forms  the  nuclear  substance  is 
diffused  through  the  protoplasm  (Gruber). 


28    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

in  which  a  covering  or  test  is  developed  ;  this  test 
may  be  chitinous  (Gromia),  or  chitinous  and  calca- 
reous, or,  in  rare  cases,  siliceous;  and  it  may  have  either 


Fig.  2  c. — Xiphacs/ntha,  showing  the  siliceous   skeleton.      (After  W. 
TJUOIUSOH.) 

a  single  large  orifice  (Fig.  2  A),  or  the  test  may  be  per- 
forated  with    a  number  of  holes    (Foraminifera), 

and  may  attain  to  a  large  size  (Nummulites),  and  great 
complexity  of  form. 


Chap.  III.] 


GROUPS  OF  PROTOZOA. 


29 


The  Heliozoa  either  have  the  body  naked  or  a 
siliceous  skeleton  is  developed ;  the  body  is  very  com- 
monly spherical  in  shape,  while  the  pseudopodia 
(Fig.  2  B)  are  fine,  alter  but  little  in  form,  and  rarely 
anastomose  with  one  another ;  lastly,  the  Radiolaria 
(Fig.  2  c)  have  a  chitinous  "  central  capsule,"  around 
which  flows  the  protoplasm,  and  with  which  there  is 


Fig.  3  I. — Paramcecium  aurelia  ;  A,  from  the  side ;  B,  from  below  ;  c,  two  in 

conjugation, 
n,  Nucleus  ;  b,  mouth  ;  cv,  contractile  vacuole. 

ordinarily  connected  a  delicate  and  often  elaborate 
siliceous  skeleton.  The  pseudopodia  are  less  constant 
in  form  than  in  the  Heliozoa,  and  enter  into  anasto- 
moses with  their  neighbours. 

The  Infusoria  are  ordinarily  ciliated,  but  in 
some  (Flagellata)  the  cilia  are  replaced  by  a  single 
long  whip-like  process  of  protoplasm  (flagellum) 
(Fig.  3  ii.),  and  in  others  which  are  parasitic  on 
(ectoparasitic)  the  bodies  of  other  infusorians,  the 
cilia  are  lost  and  replaced  by  tentacle-like  sucking 
tubes  (Fig.  3  in.). 


30    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

I.  Ciiiata,  as  Paramoecium,  Vorticella,  and 
others ;  the  cilia  are  either  regularly  distributed  over 
the  cell,  and  are,  for  the  most  part,  subequal  in  size 
(Paramoecium)  (Fig.  3  i.)  ;  or  some  are  much  larger 
than  the  rest  (Stentor) ;  or  the  cilia  are  ordinarily  con- 
fined to  a  spiral  circlet  around  the  mouth  (Vorticella), 
and  are  only  occasionally  found  on  other  parts  of  the 


an 


Fig.  3  ii.— A,  Noetiluca  miliaris ;  B,  with  buds  ;  c,  section. 
n,  Nucleus  ;  /,  flagellura ;  t,  tentacle ;  d,  denticle ;  an,  anus. 

body ;  or,  finally,  they  may  be  limited  to  the  so-called 
ventral  surface  (Euplotes) ;  in  the  Peritricha,  as  the 
group  to  which  Vorticella  and  its  allies  belong  is  called, 
there  is  often  an  elongated  aboral  stalk,  which  some- 
times exhibits  a  remarkable  power  of  rapid  contraction. 
II.  Flagellata ;  a  number  of  forms  are  grouped 
by  some  writers  under  this  head  ;  of  such  as  are 
almost  indubitably  animal,  Noetiluca  (Fig.  3  n.),  the 
animalcule  which  causes  much  of  the  diffused  phos- 
phorescence of  the  sea,  is  one  of  the  best  known. 


Chap.  III.] 


METAZOA. 


Fig.    3    in.  —  Acineta 
tuberosa. 


III.  Suctoria:  in  these  parasites  (e.g.  Acineta, 
Fig.  3  in.),  the  mouth  is  lost  and  the  sucking  tubes 
protruded  from  the  protoplasmic 
mass  serve  to  convey  food  into 
the  body.  A  study  of  their 
development  reveals  the  interest- 
ing fact  that  they  commence  life 
as  ciliated  embryos,  and  suggests 
the  idea  that  they  are  descended 
from  ciliate  infusoria. 

The  Sporozoa  will,  for  the 
purposes  of  this  book,  be  repre- 
sented by  the  Gregarinida.  The 
forms  best  adapted  for  study  are 
the  gigantic  Gregarine  found  in 
the  intestine  of  the  lobster,  and 
remarkable  for  being,  though  but 
a  single  cell,  as  much  as  two -thirds  of  an  inch  in 
length  •  and  the  much  smaller  species  found  in  the 
testicular  reservoirs  of  the 
earthworm. 


II.— THE  METAZOA. 
-C      STRUCTURE     AND     EARLY     HIS- 
TORY   OF    THE     EGG-CELL. 

The  key  to  the  structure 
of  the  higher  animals,  or 
Metazoa,  is  to  be  found  in 
a  knowledge  of  the  early 
history  of  the  egg  from 
which,  as  has  been  already  said,  they  all  arise. 
This  cell,  when  mature,  consists  of  a  mass  of  proto- 
plasm (Fig.  4,  c),  with  a  central  nucleus  (b),  and  con- 
tained nucleolus,  and  in  most,  though  not  in  all  cases 
(Hydra),  it  has  a  definite  investing  membrane  (a). 
Under  normal  circumstances  this  egg-cell  is  fertilised 


Ripe 
(Afte 


er  Klein.) 
a,  Envelope  ;  b,  nucleus  ;  c, 
protoplasm. 


32    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

by  the  male  element  (chap,  xiii.),  and  then  commences 
to  undergo  a  process  of  cleavage,  or  division.  It  first 
divides  into  two  cells,  which  are,  in  the  simplest  cases, 
equal  in  size  ;  each  of  these  again  divides,  so  that  there 


Tig.  5. — Segmentation  of  Amphioxus. 

A,  Stage  with  two  equal  segments;  u,  with  four;  c,  with  eight;  D,  segments 
enclosing  a  segmentation  cavity  ;  E,  somewhat  c  Ider  stage  in  optical  section. 
(After  Kowalevsky.) 

are  four,  then  eight,  and  so  on.  After  a  time  the  pro- 
cess of  segmentation  (Fig.  5)  comes  to  an  end,  and 
then  we  have  a  mass  of  segments,  which  are  either 
closely  applied  to  one  another,  and  so  have  a  kind  of 
mulberry-like  appearance  (hence  the  name  of  momla 
applied  to  this  stage) ;  or,  as  is  more  common,  the 
segments  separate  from  one  another  during  the 
process  of  division,  and  give  rise  within  to  a  space, 


Chap.  TIL] 


THE  GASTRULA. 


33 


the  segmentation  cavity;  the  cells  bounding 
this  cavity  then  undergo  a  further  change,  by  means 
of  which  the  single  becomes  replaced  by  a  double 
layer,  one  of  which  is  interior  to  the  other. 

This  two-layered  condition  is  brought  about  in 
one  of  two  ways ;  either  the  cells  of  one  half  of  the 
sphere  are  pushed  into  the  contained  space,  and,  by 
approaching  the  other  half,  more  or  less  completely 
obliterate  the 
segmentation 
cavity,  or  the 
cells  undergo  a 
transverse  and 
concentric  clea- 
vage, by  means 
of  which  each 
cell  becomes 
two,  and  the 
single  is  con- 
verted into  a 
double  layer. 
Whether  the 
former  process 
(that  of  iii- 
vagi  nation) 
or  the  latter  (delamiiiation)  takes  place,  the  cell- 
layers  are  regarded  as  comparable,  and  receive  the  same 
names ;  the  outer  is  known  as  the  epiblast  (Fig.  6,  ep), 
the  inner  as  the  hypotolast  (hyp).  Similarly  the  con- 
tained cavity,  which  is  clearly  the  segmentation 
cavity  in  the  latter  mode,  and  an  altogether  new 
formation  in  the  former,  is  spoken  of  as  the 
arclieiiteroii,  while  the  narrow  opening  to  the  ex- 
terior is  the  blastopore  (o).  The  whole  organism  is 
now  said  to  be  in  the  Gastmla  stage  (Fig.  6). 

No  known  animal  remains  at  quite  the  low  and 
undifferentiated  condition  of  a  Gastrula ;  and,  indeed, 
D— 16 


Fig.  6. — Diagram  of  a  Gastrula. 

o,  Blastopore;  ep,  epiblast ;  hyp,  Lypoblast, 


34    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

in  most  cases  yet  another  germinal  layer,  as  the 

epiblast  or  hypoblast  is  respectively  called,  is  developed 
between  the  two  we  know  already.  It  is  appro- 
priately spoken  of  as  the  mesol*last;  it  arises  in 
various  modes,  into  the  distinctions  of  which  we  need 
not  enter  here  ;  it  will  suffice  for  us  to  know,  that 
in  all  the  higher  Metazoa  the  greater  part  of  the 
organism  is  fashioned  out  of  it. 

In  all  cases  the  outer  and  inner  layers  undertake 
the  functions  which  their  position  entails  on  them  ; 
the  cells  of  the  epiblast  become  converted  into  the 
parts  which  cover  in  and  protect  the  rest  of  the  body, 
and  give  rise  also  to  those  organs  by  means  of  which 
the  organism  becomes  acquainted  with  what  is  going  on 
around  it,  sensory  organs  and  nervous  system. 
The  hypoblast  remains  always  in  connection  with  the 
enteron,  or  digestive  tract,  forming  the  lining  of  its 
walls,  of  the  glands  that  are  therein  developed,  and  of 
such  outgrowths  as  may  arise  from  it.  In  the  lower 
divisions  of  the  Metazoa  the  mesoblast  does  not  take 
any  large  share  in  the  formation  of  the  organs  ;  it 
remains  in  a  more  or  less  indifferent  condition.  In 
the  higher  forms  it  becomes  quite  the  most  important 
layer  in  the  body,  taking  on  as  it  does  the  duty  of 
developing  the  skeleton,  the  muscles,  the  blood,  and 
vascular  system,  the  excretory  organs,  and  the  con- 
necting tissues ;  it  always,  also,  becomes  primarily 
cleft  or  divided,  so  that  a  cavity  is  developed  within 
it ;  this  is  the  true  body  cavity,  or  ccelom,  and  all 
animals  that  possess  it  may,  whether  they  secondarily 
lose  it  or  not,  be  spoken  of  as  the  C«plomata. 

The  acoalomate  Metazoa  are  the  sponges  (Pori- 
fera),  and  the  great  group  to  which  belong  hydra, 
the  jelly-fishes,  and  the  sea-anemones  (Ccelenterata). 
The  simplest  sponges  show  hardly  any  advance  on 
the  typical  Gastrula,  the  amount  of  mesoblastic  tissue 
developed  being  small;  but  they  are  remarkable  at 


Chap.  III.] 


SPONGES. 


35 


once  for  a  character  which  sharply  distinguishes  from 

all   other   animals.      It   happens  to    many  Gastrulse 

that,    their   blastopore  closing  up,   they   develop    an 

investment  of  cilia  on  their  epi- 

blast,    and    swim    about    for   a 

time   freely   in   the  water;  but 

these   cilia  are  confined   to   the 

outer    surface.     In  the    sponges 

it  is  otherwise,  the  ciliated  cells 

early  become  internal  to  the  non- 
ciliated,  and  some  are  retained 

throughout  life  in  the  so-called 

"  ciliated  chambers."     When  we 

come  to  examine  into  the  activity 
of  a  living  sponge  we  find  no 
advance  on  that  of  a  Protozoon, 
save  so  far  as  the  division  of 

labour  is  here  first  clearly  seen  ; 

we  find,  that  is,  that  the  multi- 

celluiai\  organism   feeds,    grows, 

respires,    reproduces    itself,    and 

dies  ;  and  we  find,  too,  that,  like 

many    Protozoa,     it    forms     for 

itself  firm  supports  in  the  way 

of   a  skeleton,   but  we   find   no 

cells   that  are  specially  sensory, 

and    none    that     are    obviously 

muscular ;  there    is  the  general 

irritability       and       contractility 

which  living  protoplasm    always    exhibits,  but  there 

are  no  special  organs  for  either  function. 

The  Porifera,  or  sponges,  fall  into  the  following 
divisions  : 

1.  Ittyxospoiigiae,  in  which  there   is    no  hard 
skeleton  ;  e.g.  Halisarca. 

2.  Calcispoiigiae,  in  which  a  support   for  the 
body  is  furnished  by  calcareous  spicules ;    e.g.  Ascon, 


Fig.  7.  — Calcareous 
Sponge.  AKcetta  pn'mor- 
ctoa7f*.  (After  Haeckel, 
x  SOdiams.) 


36    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Leucon,    Sycon.       The    commonest    British   form    is 
ordinarily  known  as  Grantia.     (See  Fig.  7.) 

3.  Silicispongiae,  in  which  part  of  the  skeleton 
is  made  up  of   spicules  of  silica ;    e.g.    the  common 
fresh-water  sponge  (Spongilla),  Chalina,  Euplectella. 

4.  Ceratospongise,   in   which   the   skeleton   is 


Fig.  8. — A,  Hydra  v 'ridis,  attached  to  Duckweed ;  B,  a  Single  Specimen 

magnified  ;  c,  Hydra  in  Diagramatic  Section, 
cc,  Ectoderm  ;  en,  endodenu  ;  TO,  mouth  ;  be,  enteric  cavity  ;  t,  tentacle?. 

completely  horny  or  fibrous,  and  devoid  of  siliceous  or 
calcareous  spicules  ;  e.g.  the  bath-sponges  (Euspongia). 

In  the  Cvceleiiterata  it  is  otherwise;  in  many 
forms  both  nervous  and  muscular  tissues  are  to  be 
recognised  not  only  by  the  aid  of  the  microscope, 
but  by  the  activity  of  these  animals,  and  by  their 
reactions  when  subjected  to  physiological  experiment. 

Henceforward,  then,  we  have  to  do  with  forms 
which  possess,  in  some  shape  or  other,  all  the 
essential  tissues  of  even  the  most  complicated 


Chap.  III.]  CCELENTERATA.  37 

organisms  ;  differentiation  will  lead  to  greater  sub- 
division of  labour,  and  greater  complexity  of  struc- 
ture, but  all  the  materials  are,  even  so  low  in  tlie 
grade  of  animal  life,  ready  to  our  hand. 


"Fig.  9. — Perigonimus  vestitus,  showing  Trophosomes  and  Gonosoines 
(After  Allman.) 

A  ccelenterate  animal,  then,  is  one  in  which  the 
archenteron  of  the  gastrula,  even  when  secondary 
outgrowths  are  developed  from  it,  remains  always  as 
the  only  cavity  in  the  body,  in  which  the  mesoblast 
is  but  imperfectly  differentiated,  but  in  which  organs 
of  offence,  locomotion,  and  sensation  are  added  on  to 
the  structures  of  the  original  gastrula  form. 


38    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


In  its  simplest  known  condition,  e.g.  Hydra 
(Fig.  8),  a  Coelenterate  has  a  terminal  mouth  (ra) 
which  leads  into  a  digestive  cavity  (be),  and  around 

which  ten- 
tacles (t)  are 
developed; 
these  tentacles, 
which  serve  as 
organs  of  pre- 
hension, sen- 
sation, and 
offence,  are 
hollow,  con- 
tinuations of 
the  enteric 
cavity  passing 
into  them. 
There  is  no 
second  orifice 
to  the  enteron, 
and  reproduc- 
tion is  effected 
either  by  gem- 
mation, or  by 
the  formation 
of  ova  and 
spermatozoa. 

In  the  more 
complicate  d 
members  of 

the  group  the  hydriform  body  gives  off  buds,  and  be- 
comes one  of  a  colony  (Fig.  9) ;  and  the  separate 
"  persons  "  of  this  colony  are  connected  together  by  a 
common  trunk,  which  is  hollow  within,  and  continuous 
with  the  enteric  cavity  of  each  person ;  in  the  simplest 
stage  of  these  colonial  formations  each  person  performs 
the  same  duties,  but  in  the  more  complex  different 


Fig.  10.— Figure  of  the  Medusa  of  a  Hydroid. 
(After  Hincks.) 


Chap.  III.] 


MEDUSA. 


39 


persons  take  on  different  duties;  when  these,  again, 
are  at  their  simplest  stage,  we  find  that  while  some 
nourish  the  colony  (tropliosomes),  they  take  no 
share  in  reproducing  it;  this  office  is  performed  by 
other  persons  (gonosomes),  which  depend  for  their 
nourishment  on  the  neighbouring  tropliosomes.  Di- 
vision of  labour  among  the  persons  of  the  colony  may 
go  still  farther,  and  groups  become  formed  of  which 
some  have  nutrient,  others  locomotor,  others  protective, 
and  others  prehensile  or  offensive  functions  (Siphono- 
phora;  e.g.  Portuguese  man-of-war)  (Fig.  12).  Where 
the  Ccelenterate  is  fixed,  we  observe,  in  one  division, 
that  the  generative  persons  become  free-swimming, 
and,  while  retaining  the 
essential  characters  of  the 
division,  become  greatly 
altered  in  form,  in  adap- 
tation to  their  new  mode 
of  life ;  such  persons  are 
spoken  of  as  Medusae 
(Fig.  10).  Finally,  we 
find  that,  in  some  cases, 
the  fertilised  ovum  of  a 
medusa  gives  rise  not  to 
a  fixed  hydra-like  body, 
but  directly  to  a  medusa 
form.  The  tentacles  are 
set  round  the  mouth  in  a 
circle,  and  the  parts  of 
the  body  are  similarly 
arranged  in  a  fashion  of 
symmetry,  which  is  called 
radial ;  where,  however, 
the  free  mode  of  life  has  obtained  for  a  long  period  of 
time,  we  sometimes  find  that  there  is  only  one  axis  of 
the  body  on  either  side  of  which  exactly  corresponding 
parts  are  to  be  found ;  in  other  words,  a  bilateral 


Fig.  11.  — Longitudinal  section 
through  Sagartia  parasitica, 
showing  a  meseuteric  septum 
with  the  body  wall  to  the  right, 
and  the  enteric  wall  to  the  left. 
(After  O.  and  E.  Hertwig.)  (See 
Fig.  54.) 


4o    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

takes  the  place  of  a  radial  symmetry ;  e.g.  Venus'  girdle 
among  the  Otenophora.     (See  Fig.  16,  page  46.) 

The  Cceleiiterata  fall  into  two  well-marked 
divisions,  Hydrozoa  and  Anthozoa ;  in  the 
former  the  mouth  is  placed  011  a  projecting  oral  cone, 
while  in  the  latter  it  is  sunk  below  the  level  of  the  oral 
circlet  of  tentacles,  and  the  cavity  developed  from  the 
enteron,  and  separating  its  wall  from  the  body  wall, 
is  traversed  by  partitions  (meseiiteric  septa) 
(Fig.  11),  of  which  a  certain  number  extend  across 
the  whole  of  the  cavity,  while  others  only  project  for 
a  shorter  or  longer  distance  into  it. 

CCELENTERATA. 

A.  Hydrozoa.— The  hydrozoa  fall  into  two  well- 
marked  divisions,  in  the  first  of  which  the  medusa 
form,  when  developed,  always  has  an  infolded  rim  of 
the  body  running  round  the  inner  edge  of  the  mouth 
of  the  bell  (velum).  In  consequence  of  the  presence 
of  this  fringe  it  may  be  spoken  of  as  the  Craspedote 
division ;  in  it  the  sense  organs  are  never  protected 
by  any  lid  or  cover,  and  they  are  therefore  known  as 
the  Naked-eyed  Medusse  (Oymnophthalinata), 
and  as  the  generative  sacs  never  form  projecting 
pouches,  they  are  by  some  spoken  of  as  Cryptocarpa. 

I.  Craspedota.— The  Craspedota  fall  into  three 
groups ;  in  the  first  the  organism  is  always  hydri- 
form. or  the  nutrient  persons  are  hydriform,  and 
the  generative  medusiform,  or  the  organism  is  always 
medusiform.  They  may,  therefore,  be  called  Hydro- 
medusae.  Examples  of  these  are  :  Hydra,  Cordylo- 
phora,  Hydractinia,  Sarsia,  Oceania. 

In  the  second  group  we  have  those  colonies  of 
hydriform  persons  in  which  the  common  stem  becomes 
richly  impregnated  with  calcareous  salts,  and  they 
therefore  may  be  known  as  Hydroid  Corals  or  Hydro- 
<  01  alliii;r.  Such  are  Millepora  and  Stylaster. 


Chap.  III.] 


JELLY-FISHES. 


In  the  third  group 
we  have  those  free- 
swimming  colonies  to 
which  reference  has 
already  been  made  as 
examples  of  the  highest 
form  of  division  of 
labour  :  they  are  called 
the  Siphonophora, 
and  Velella,  Diphyes, 
Physalia,  and  Physo- 
phora  (Fig.  12)  belong 
to  this  group. 

Scypliomedusse.— 
In  the  second  great 
division  of  the  hydrozoa 
we  have  the  forms 
which  are  best  known 
as  the  Medusae,  or  jelly- 
fishes  par  excellence. 
With  one  exception, 
they  all  pass  through 
a  stage  which,  at  first 
somewhat  hydriform  in 
appearance  (Scypliis- 
toma-stage),  is  re- 
markable for  under- 
going transverse  di- 
vision ;  each  of  the 
segments  so  formed 
separates  and  forms  an 
independent  medusa. 
When  adult  they  are 
always  medusiform  in 
appearance,  and,  as  they 
rarely  have  a  velum  to 
their  disc,  they  are 


Fig.  12. — PhysopTiora  hydrostatica. 

a,  Air-bladder;  m,  nectocalyx  ;  jjr,  gener- 
ative persons;  ?».,  nutrient,  persons  (in 
the  form  of  sucking  tubes) ;  t,  tentacu- 
lar persons.  (After  Cuvier.) 


42   COKJ-ARAWE  ANATOMY  AND  PHYSIOLOGY. 


£.  13.-- Aurelia  aurita. 


large   and   obvious       Th  «eneptive  glands  are 


Chap.  III.] 


ANTHOZOA. 


43 


an  example  of  the 
forms  in  which  the 
original  mouth  is  lost, 
and  replaced  by  a 
number  of  small  aper- 
tures developed  on  the 
long  arm  -  like  out- 
growths of  its  lips. 

B.  Anthozoa.— 
Among  the  Anthozoa 
we  find  the  sea- 
anemoiies  and  the  great 
bulk  ol  those  cceleri- 
terates  which  form 
coral. 

According  as  they 
possess  eight,  and  eight 
only,  or  six,  or  some 
multiple  (often  a  large 
one)  of  six,  we  divide 
the  Anthozoa  into  the 
Octactiniae,  and  the 
HexactiiiisB. 

I.  The  Octactinise 
have  never  more  than 
eight  tentacles,  and 
these  are  flattened  and 
serrated  at  their  edges. 
In  Alcyonium  ("dead 
men's  fingers ")  cal- 
careous spicules  are 
scattered  in  the  body  ; 
in  Tubipora  ("organ- 
pipe  coral")  the  spi- 
cules collect  and  form 
a  continuous  tube  for 
each  polyp  (Fig.  14  A)  ; 


Fig.    14  B.— Pennatula 
(Pteroides)  spinosa. 


44    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

in  the  sea-pen  (Pennatula)  (Fig.  14  B),  the  tissue  which 
connects  the  polyps  together  is  horny,  in  the  noble  red 


Fig.  14  c. — Gorgonia  fldbellum. 

coral  it  is  calcified,  while  in  the  sea-fans  (Gorgonia)  an 
elegant  hard  network  is  developed  (Fig.  14  c). 

II.  The  Hexactinise ;  the  six  tentacles  or  mul- 
tiples of  that  number  are  filiform,  and  their  edges 
smooth.  Some,  like  the  common  sea-anemone, 
remain  single  throughout  life,  but,  in  most,  buds  are 
given  off,  and  a  colony  is  formed.  The  deposition  of 


Chap.  III.] 


CTENOPHORA. 


45 


calcareous  salts  often  gives  rise  to  large  masses  of 
"  stony  "  coral,  of  which  the  brain-coral  (Mseandrina) 
is  a  good  example ;  in  other  cases  (e.g.  Fungia)  ths 
septa  are  alone  calcined. 

There  still  remains  a  division  of  the  Ccelenterata 
which,  though  it  has  been  definitely  placed  by  some 
naturalists  with  the  Hydrozoa, 
and  by  others  with  the  Antho- 
zoa,  is  possibly  an  independent 
group  ;  in  these,  the  eight  canals 
derived  from  the  enteron  run 
at  equal  distances  close  to  the 
surface  of  the  body,  and  along 
these  there  are  formed  bands  of 
cilia,  which  have,  in  consequence 
of  their  comb-like  appearance, 
gained  for  these  forms  the  name 
Ctenopliora.  The  glassy  globe 
called  Cydippe  (Fig.  15)  is  found 
on  our  own  shores,  while  Venus' 
girdle  (Cestus  veneris)  is  an  ex- 
ample of  that  acquired  bilateral 
symmetry  to  which  we  have  already 
referred  (Fig.  16). 


Fig.  15.— Cydippepileus. 


THE    HIGHER    METAZOA. 

In  the  remaining  Metazoa  a  cavity  distinct  from 
the  archenteric  cavity  becomes  developed,  and  the 
mesoblast  becomes  the  seat  of  those  important  changes, 
by  means  of  which  nearly  all  the  tissues  of  the  body 
are  derived  from  it.  In  the  midst  of  this  mesoblast 
a  cavity  arises  by  cleavage  or  fissure,  or  from  the 
archenteron  there  are  given  off  out-growths  which, 
in  time,  become  shut  off  from  the  parent  space,  and 
occupy  the  middle  of  the  mesoblast.  The  cavity 
formed  in  either  of  these  ways  is  spoken  of  as  the  body 
cavity  or  ceelom,  and  the  result  of  its  appearance  is 


46    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

that  the  mesoblast  becomes  separated  into  two  layers, 
one  of  which  applies  itself  to  the  epiblast,  and 
the  other  to  the  hypoblast ;  in  this  way  we  get 
the  somatopleure  and  splanclmopleure  of 


Fig.  16.— Venus'  Girdle  (CeJus  veneri*), 

embryologists.  All  the  Metazoa  that  possess  this 
body  cavity  may  be  spoken  of  collectively  as  the 
Cflelomata.  In  some  cases  the  coelom  remains 
throughout  life  in  a  very  rudimentary  condition,  and  in 
a  few  it  cannot  be  said  to  be  developed  at  all,  while  in 
others  it  would  seem  to  have  been  lost  by  degeneration. 
According  to  its  mode  of  origin,  as  an  out-growth 
from  the  enteron,  or  by  cleavage  of  the  mesoblast,  it 
is  spoken  of  as  an  enteroccele,  or  a  scliizocoele. 


Chap,  in.]  METAZOA.  47 

The  archenteron  ordinarily  closes  up,  so  that  the 
blastopore  disappears  ;  a  fresh  mouth,  and  in  most 
cases  also,  an  anus,  are  developed  at  either  end  of 
the  tube  ;  these  are  lined  by  inpushings  of  the  epi- 
blast;  the  epiblastic  pits,  deepening  and  elongating, 
finally  become  continuous  with  the  original  or  arch- 
enteric  cavity,  which  is,  it  wil]  be  remembered, 
lined  by  hypoblastic  cells.  In  a  fully  developed  di- 
gestive tract  we  have  now  to  distinguish  three  regions  : 
(1)  a  mouth  passage  (stomodceum)  which  is  lined 
by  epiblast  ;  (2)  a  mid-intestine  (mesenteroii)  lined 
by  hypoblast ;  and  (3)  an  anal  passage  (procto- 
<8<rum)  lined  again  by  epiblast. 

The  greater  number  of  the  Metazoa  are  free 
animals,  and  no  doubt  the  ancestors  of  all  the  terres- 
trial were  aquatic  forms;  organisms  moving  freely 
in  such  a  medium  as  water  would  clearly  have 
one  end  which  was  anterior  and  one  which  was  pos- 
terior, and  as  these  would  be  differently  affected  by 
the  water  through  which  they  moved,  the  one  end 
wrould  become  differently  constituted  to  the  other ;  the 
anterior  end  would  be  that  at  which  food  would  be 
taken  in,  and  at  which  the  prey  or  an  enemy  would 
l>e  first  met.  This  end  would  then  be  primarily  the 
sensitive  end,  and  we  find  that  it  is  here  that  sense 
organs  of  various  kinds  are  best  developed.  In  other 
words,  we  have  henceforward  to  look  for  a  definite  re- 
gion, specially  sensitive  in  function,  developed  in  front 
of  the  mouth  ;  this  may  be  called  the  praBStomiiim. 

On  either  side  of  the  moving  body  the  water  would 
exert  equal  pressure,  and  the  two  sides  would  come  to 
exhibit  similar  characters,  or  bilateral  symmetry 
would  become  apparent.  In  shallow  waters  one  as- 
pect of  the  body  would  be  more  exposed  to  the  in- 
fluence of  light  than  the  other,  and  we  should  there- 
fore distinguish  between  an  upper  or  dorsal  and  a 
lower  or  ventral  surface. 


48    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  origin,  then,  of  the  higher  Metazoa  is  to  be 
looked  for  in  an  animal  in  which  an  anterior  end  with 
a  prsestomium  is  to  be  distinguished  from  a  posterior 
end  ;  in  which  the  two  sides  are  similar  to  one  another, 
and  the  dorsal  slightly  different  from  the  ventral 
surface.  Forms  of  this  kind  are  still  to  be  found 
among  the  lowest  Worms. 

Various  organs  must,  of  course,  be  developed  with- 
in such  an  organism  ;  in  the  simplest  cases  some  of 
the  cells  of  the  hypoblast  retain  the  power  possessed 
by  the  Amoeba  of  taking  solid  food  into  the  substance 
of  their  own  bodies ;  the  organism  being  small,  no 
special  means  of  circulating  the  nutriment  thus 
obtained  are  required ;  and,  just  as  in  the  Amoeba, 
respiration  is  carried  on  by  the  general  surface  of  the 
body,  and  by  the  water  brought  in  with  the  food.  On 
the  other  hand,  even  in  Amoeba,  we  found  a  con- 
tractile vacuole,  and  we  may,  therefore,  well  suppose 
that  in  this  complex  of  cells  there  must  be  some 
special  means  for  the  removal  from  the  body  of  its 
waste  nitrogenous  products.  At  any  rate,  the  meso- 
blast  is  on  either  side  channelled  by  a  delicately 
walled  canal  which  has  openings  into  the  spaces  in 
the  mesoblast,  and  communicates  by  a  pore  with  the 
exterior.  As  the  organism  is  to  give  rise  to  cells  from 
which  other  organisms  are  to  arise,  some  part  of 
its  body  must  be  set  apart  as  generative  cells  ;  in 
the  simplest  cases  these  are  mere  masses  of  cells  in 
simple  pouches,  which  pass  directly  into  the  water. 

Of  the  cells  in  the  region  of  the  praestomium 
some  will  be  more  particularly  modified  for  the 
reception  of  impressions  from  the  outer  world,  and  will 
form  a  rudimentary  nervous  mass,  with  which  a  few 
nerve-fibres  will  be  connected ;  as  the  creature  is 
capable  of  moving  from  place  to  place,  we  have, 
further,  to  look  for  the  presence  of  muscular 
tissue. 


Chap,  in.]  FLAT- WORMS.  49 

The  lowest  Metazoa  are  grouped  into  a  somewhat 
heterogeneous  mob,  which  is  known  as  the  division  of 
the  Verities  or  Worms.  Of  these  the  lowest  are  the 
Flat-Worms. 

A.  Platyhelmintlies. — Of  the  three  divisions  of 
flat-worms,  two  are  degraded  by  parasitism  ;  such  are 
the  divisions  to  which  the  tape-worms  (TaBnia),  and 
the  flukes  (Distomum)  belong. 

I.  The  Turfoellaria  are  the  simplest  forms,  and 
are   free  living;  the  body  is  soft  and  small,  covered 
with  cilia,  and  without  an  anus ;  the  entrance  to  the 
digestive  tract  is  often  provided  with  a  proboscis,  and 
the  generative  apparatus  may  be  simple,  or  may  be 
greatly  complicated.     A  distinct  crelom  is  not  always 
apparent    (Accela),    or   it   may   become    secondarily 
obscured.     The  enteric  tract  is  straight,  or  branched. 
Planaria,    Dendrocoelum,    and    Mesostomum  are  ex- 
amples of  this  division. 

II.  The  Trematoda  are  flat-worms  that  have 
taken   to  a  parasitic  mode    of   life,    but   are    by  no 
means  so  profoundly  modified  as  the  members  of  the 
group  next  to   be  considered.     They    either  live  on 
the    bodies    of    other    animals    (ecto-parasitic),    like 
A spidogaster,  which  is  found  in  the  gill  chamber  of  the 
fresh-water    mussel ;    in    this    case   they   exhibit   no 
"  alternation  of  generation."     Or  they  live  within  the 
bodies   of  other  animals   (pento-arasitic),  like  Disto- 
mum hepaticum  (the  liver-fluke) ;    in  this  case   they 
pass  different  stages  of  their  existence  in  two  different 
animals.     The  ciliated  covering  is  lost,  and  suckers  are 
developed,  which  serve  as  organs  of  attachment,  and, 
to  a  certain  degree  also,  as  organs  of  locomotion ;  the 
sexes  are  ordinarily  united  in  the  same  individual,  and 
the  accessory  parts  of  the  generative   apparatus  are 
greatly  complicated. 

III.  The    Cestoda,    or     tape-worms,     are     flat- 
worms  which  are  still  further  modified  in  accordance 

E— 16 


50    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

with  their  constantly  eiito-parasitic  habit  of  life,  and 
they,  like  the  endo-parasitic  Trematoda,  ordinarily  pass 
through  different  stages  of  their  development  in 
different  hosts.  While  the  simplest  forms,  like  the 
Caryophyllaeus  of  the  carp,  exhibit  no  kind  of  jointing 
or  division  of  the  body,  and  Ligula  has  the  jointing 
affecting  only  the  internally  placed  generative  organs, 
most  consist  of  a  more  or  less  large  number  of  joints  ; 
Tsenia  echinococcus  having  three  or  four,T.  solium  about 


Fig.   17.— Tomia,    showing  the  head  and  four   suckers, 
the  unjointed  neck,  and  the  early  joints  (Strohila). 


a  thousand,  and  Bothriocephalus  latus  having,  it  is  said, 
as  many  as  10,000  joints,  and  attaining  to  a  length  of 
twenty-five  feet  (Fig.  17).  As  these  joints  increase  in 
size  and  approach  maturity,  the  ova  become  fertilised, 
and  commence  to  develop ;  on  the  joints  breaking  off 
and  escaping  to  the  exterior,  the  ova  within  are  set 
free,  and  if  eaten  by  the  other  host  proper  to  the 
tape-worm,  they  go  through  the  earlier  stages  of 
their  development  within  its  body.  In  these  parasites 
the  digestive  tract  is  altogether  aborted. 

We  have  been  carried  away  by  these  degraded 
forms  from  the  general  line  of  development ;  we  return 
to  it,  however,  only  again  to  find  ourselves  confronted 


Chap.  III.] 


ROUND-  WORMS. 


with  a  group,  the  great  majority  of  the  members  of 
which  are,  in  their  sexual  state  at  least,  endo-parasitic. 
These  are  the  round-worms  or  thread-worms  (Nemato- 
lielmi  lit  lies).  They  are  remarkable,  as  compared 
with  the  soft-bodied  Turbellarians,  for  the  great 
development  of  that  horny  material  which  is,  as  chi- 
tin,  so  richly  present  in  the  integuments  of  many 
Metazoa.  The  intestine  forms  a  straight  tube,  and 
is  surrounded  by  a  comparatively  spacious  body  cavity. 
The  whole  body  is,  as 
their  popular  name  implies, 
greatly  elongated.  Examples 
of  this  group  are  Gordius, 
Ascaris,  Filaria,  and  Tri- 
china. 

More  closely  allied  to  the 
round- worms  than  to  any 
other  worms  are  the  Acan- 
thocephali,  of  which  Echi- 
norhynchus  (Fig.  18)  is  an 
example.  They  are  internal 
parasites,  which,  like  most 
tape-worms  and  flukes,  live, 
at  different  stages  of  their 
life-history,  in  different  hosts.  They  are  provided  with 
a  protrusible  proboscis,  which  is  armed  with  recurved 
hooks  of  considerable  strength. 

The  Rotatoria  or  Wheel- Animalcules  exhibit 
certain  characters  which  we  shall  again  meet  with  in  the 
larval  stages  of  some  of  the  higher  forms.  The  anterior 
end  carries  a  disc,  the  edge  of  which  is  ciliated  (this  is 
the  so-called  "wheel-organ"),  and  in  the  centre  of 
which  the  mouth  is  placed  (Fig.  19).  A  special 
apparatus  for  comminuting  the  food  is  found  in  the 
stomach.  The  "  water- vessels,"  or  organs  by  means 
of  which,  in  all  probability,  waste  nitrogenous  matters 
are  excreted,  are  very  distinct,  and  are  provided  with 


Fig.  18.— Echinorliynclius  no- 
dulatus  (nat.  size  and  en- 
larged). (After  Busk.) 


52     COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


delicate  branches  with  terminal  orifices ;  the  two 
vessels  open  into  a  special  enlargement  or  bladder,  the 
walls  of  which  are  contractile,  so  that  the  fluid  stored 
up  in  it  can  be  forced  to  the  exterior.  The  sexes,  as 
in  Nematoids,  are  ordinarily  separate,  and  the  males 
can  be  distinguished  from  the  females  by  their  smaller 
size.  Rotifer,  Brachionus,  and  Melicerta  are  examples 
of  the  Rotatoria. 

Most  of  the  forms  with  which  we  have  already 
had  to  do  are  small  in  size,  and 
it  will  have  been  noted  that,  where 
the  body  attained,  as  in  the  case 
of  certain  tape-worms,  to  a  con- 
siderable length,  that  body  was  not 
an  individual  whole,  but  was  broken 
up  into  joints  or  segments. 

In  the  great  group  of  worms 
which  we  are  now  going  to  consider, 
this  segmentation  of  the  body  is 
very  distinctly  exhibited,  and  affects 
not  only  the  external  form,  but  the 
great  majority  of  the  internal 
organs ;  this  phenomenon  becomes 
the  more  comprehensible  when  we 
learn  that  at  one  of  its  very  earliest 
stages  in  development  the  mesoblast 
itself  becomes  regularly  segmented.  In.  the  simpler 
conditions  the  segments,  which  we  will  henceforward 
call  metameres,  are,  for  the  greater  part,  exactly 
similar  in  character,  and  only  those  at  either  end  of 
the  body  differ  much  from  the  rest.  Later  on  we  shall 
see  that,  just  as  in  the  simpler  animals,  different  parts 
take  on  different  duties,  and  division  of  labour  becomes 
as  apparent  among  the  metameres  as  it  was  in  the 
various  persons  of  the  colonial  Coelenterata. 

Now,  also,  we  find  that  organs  for   which,  in  the 
smaller  and   simpler  forms,  there  was  no  necessity, 


Fig.  19.— Brachionus; 
to  show  the  Ciliated 
head-disc. 


Chap,  in.]  HIGHER  METAZOA.  53 

gradually  become  elaborated.  The  body  is  now  too 
large  be  to  able  to  do  without  an  apparatus  by  means  of 
which  the  nutrient  material  obtained  by  digestion,  or 
the  store  of  oxygen  necessary  for  the  activity  of  the 
protoplasm  of  its  constituent  cells,  may  be  carried 
about  from  part  to  part,  and  we  have  therefore  a 
system  of  circulating  vessels.  In  many,  also,  the 
iirm  covering  of  the  body  necessitates  the  develop- 
ment of  special  outgrowths  into  which  the  vessels 
pass,  charged  with  the  carbonic  acid  which  is  con- 
stantly associated  with  the  activity  of  living  proto- 
plasm ;  in  these  outgrowths  the  blood  gives  up 
carbonic  acid,  and  receives  oxygen  in  its  place  ;  in 
other  words,  a  respiratory  is  added  on  to  a 
circulatory  apparatus.  In  the  majority,  again,  the 
body  is  too  large  to  be  able  to  move  about  without 
the  assistance  of  special  muscular  processes  or  limbs, 
and  these  are  not  unfrequently  strengthened  .and  sup- 
ported by  those  chitinous  secretions  which  we  call 
setae  (bristles). 

Elaborate  and  complex  activities  of  such  a  kind 
as  these  require  to  be  brought  into  relation  with  one 
another,  or,  in  other  words,  to  be  co-ordinated, 
and  performed  in  regular  and  systematic  fashion  ;  it 
is  not  now  sufficient  for  the  organism  that  there 
should  be  a  prsestomial  nervous  mass  with  some  few 
nerve-fibres  given  off  from  it.  Centres  of  nervous 
activity  must  be  developed  in  various  parts  of  the  body, 
and  we  find,  therefore,  that  collections  of  nerve-cells 
are  found  in  different  metameres  ;  these  ganglionic 
masses  are  connected  together  by  fibres,  and  so 
it  results  that  there  runs  down  the  ventral  surface 
of  the  body  a  chain  of  ganglia.  From  each 
of  these  ganglia  nerve-fib'res  pass  to  the  muscles 
and  other  organs  of  the  body,  and  to  them  there 
come  other  fibres  which  have  one  end  in  the 
skin,  and  which  convey  to  the  central  apparatus 


54    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

some  information  of  what  is  going  on  in  the  world 
around  it. 

General  sensibility  of  this  kind  is,  however,  soon 
found  to  be  insufficient  for  the  needs  of  the  organism  ; 
sight  and  hearing  are  possessed,  no  doubt,  by  lower 
forms,  but  we  shall  soon  find  creatures  with  elaborate 
eyes,  and  well-defined  auditory  organs,  while  obscure 
indications  of  an  olfactory  sense  are,  a  little  later, 
to  be  detected. 

The  organisation  of  the  ringed  worms  or  Amui- 
lata  attains  its  highest  degree  of  complexity  in  the 
free-swimming  marine  forms.  Here  the  ringed  body 
has  on  most  of  its  metameres  a  single  or  double  pro- 
jection on  either  side  (parapodium),  from  which 
there  project  a  number  of  bristles  (set«) ;  at  the 
anterior  end,  the  tentacles  are  aided  by  a  number  of 
elongated  feelers,  and  a  pair  of  well-developed  eyes, 
and  sometimes,  too,  auditory  vesicles  are  to  be  found 
there.  The  mouth  is  provided  with  strong  horny 
denticulated  jaws,  which  are  moved  by  special 
muscles,  and  which  serve  to  break  up  the  food;  dif- 
ferent parts  of  the  digestive  tract  take  on  different 
functions,  and  pouches,  which  may  again  be  branched, 
sometimes  appear  at  the  sides.  A  fluid  circulates 
through  the  body  in  a  system  of  closed  vessels,  and 
some  of  these  vessels  have  their  walls  provided  with 
muscles  by  means  of  which  the  current,  which  is 
always  regular  in  direction,  is  propelled  onwards. 
At  the  sides  of  the  body  thin  outgrowths  of  its  wall 
serve  as  gills  (branchiae),  and  most  of  the  meta- 
meres are  provided  with  a  pair  of  coiled  tubes  which 
open  into  the  spacious  crelom,  and  also  to  the 
exterior ;  these  are  the  renal  organs  (nephridia). 

The  division  of  ringed*  worms  in  which  the  setze 
are  numerous  on  each  parapodium  is  called  the 
Polychseta;  of  these,  some,  like  the  sea-mouse 
(Aphrodite),  Polynoe,  and  Nereis  are  free-swimming, 


Chap    III.] 


CH&TOPODA. 


55 


Fig.  20.—  Tenbella  emmalina. 

and  form  the  group  of  the  Vagantia ;  others  give  up 
their  free  mode  of  life  and  settle  down  like  Sabella 
and  Serpula  into  tubes ;  in  these  TuMcolse  (Fig.  20), 


56    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  hinder  part  of  the  body  is  less  elaborately  de- 
veloped than  the  anterior,  which  can  be  protruded 
from  the  mouth  of  the  leathery,  sandy,  or  calcareous 
tube.  The  lowest  forms  of  the  division  have  no  setse 
at  all,  and  Polygordius,  which  may  be  taken  as  the  re- 
presentative of  the  Achseta,  retains  throughout  life  a 
circlet  of  cilia  at  its  anterior  end. 

In  another  and  lower  division  of  the  Annulata  we 
find  that  the  setse  are  never  more  than  eight  at  the 
most  in  each  bundle ;  and  such  forms  may  be  distin- 
guished from  the  Polychseta,  and  known  as  the 
Oligochaeta.  Of  these  the  best  known  form  is 
the  common  earthworm  (Lumbricus),  but  all  are 
not,  like  it,  terrestrial  in  habit ;  Nais  and  the 
blood-worm  (Tubifex)  are  inhabitants  of  fresh 
water. 

Most  appropriately,  perhaps,  associated  with  the 
Annulata,  but  exhibiting  a  number  of  characters  that 
bring  them  into  relation  with  the  flat- worms,  are  the 
leeches  or  H imdinea ;  living  on  the  blood  of  other 
animals,  as  many  of  them  do,  they  have  the  integu- 
ment often  developed  at  one  or  two  points  into 
suckers,  by  means  of  which  they  attach  themselves 
to  other  animals,  or  to  firm  bodies,  from  which  they 
can  extend  themselves  to  seize  or  attach  themselves 
to  their  prey. 

Most  closely  allied  to  the  Annulata,  but  best  kept 
in  a  separate  division,  are  those  marine  worms  of 
which  Sipunaulus  is  the  best  known  example ;  for 
these  the  old  term  of  Oepliyrea  may  be  retained, 
without  prejudice  to  our  views  of  the  value  of  the 
ideas  which  gave  rise  to  the  name.  The  body  ex- 
hibits no  external  segmentation  ;  they  are  remark- 
able for  possessing  excretory  organs  of  the  kind 
found  in  the  Annulata,  as  well  as  those  seen 
in  Rotifers ;  in  some  cases  the  anus  is  not  at 
the  hinder  end  of  the  body,  but  the  intestine  is  so 


Chap.  III.]         GROUPS  OF  HlGHER  METAZOA.  57 

coiled  on  itself  that  its  orifice  comes  to  lie  at  the 
side,  and  in  the  anterior  half  of  the  body. 

The  difficulties  arising  from  our  imperfect  know- 
ledge, and  the  generalised  characters  of  the  lower 
forms  which  are  associated  together  under  the  head  of 
the  Vermes,  disappear,  for  the  most  part,  when  we 
rise  above  them  in  the  scale  of  animal  organisation. 

No  one,  for  example,  can  fail  to  see  that  a  starfish 
is  no  close  ally  of  a  crayfish,  or  a  snail  of  a  frog ;  on 
the  other  hand,  a  sea-urchin  and  a  starfish  are  as 
clearly  allied  to  one  another  as  is  the  crayfish  to  the 
crab,  the  mussel  and  snail  to  the  octopus,  and  the 
shark  to  the  frog,  the  pigeon,  or  the  rabbit. 

While  the  bases  or  origins  of  these  several  forms 
are  obscure  enough,  the  apex  stands  sharply  out,  and 
we  may  compare  the  four  series  of  forms  of  which 
mention  has  just  been  made  to  four  great  branches 
arising  from  a  common  trunk.  Each  of  these  branches 
may  be  called  a  phylum.  In  one  the  body  wall 
becomes  richly  impregnated  with  calcareous  salts, 
which  sometimes  form  projecting  spines,  the  original 
bilateral  symmetry  yields  to  an  acquired  radial  one, 
and  locomotion  is  typically  effected  by  a  special  series 
of  suckers  connected  with  a  system  of  water-tubes ; 
this  is  the  phylum  of  the  Echinodermata  or  star- 
fishes. In  another  the  soft  body  becomes  invested  in 
and  protected  by  a  hard  shell  which  is  secreted  by  a 
special  outgrowth  of  the  body  called  the  mantle  ; 
the  ventral  surface  is  drawn  out  into  a  muscular  foot, 
and  a  series  of  delicate  filamentous  processes  grow  out 
on  either  side  of  the  body ;  this  is  the  phylum  of  the 
Mollusca,  or  shell-fish. 

In  yet  another  series  we  find  a  closer  resemblance 
to  the  Annulata  than  is  exhibited  in  any  other  of  the 
higher  phyla.  Some  or  all  of  the  metameres  become 
provided  with  appendages,  which  are  most  often 
jointed,  and  one  or  more  of  these  pairs  of  appendages 


58    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

become  specially  modified  to  the  purpose  of  the 
mouth.  This  phylum,  which  we  will  call  that  of  the 
Arthropoda,  might,  if  constancy  of  nomenclature 
were  not  a  matter  of  convenience,  be  more  appro- 
priately designated  as  the  Cnathopoda  (Lankes- 
ter).  Lastly,  there  is  an  important  phylum  for 
which,  in  the  light  of  recent  researches,  it  seems  well 
to  adopt  some  other  name  than  the  ordinary  desig- 
nation of  Vertebrata.  This  phylum  is  remarkable 
for  the  development  along  the  dorsal  area  of  a  rod, 
which,  at  first  hollow,  subsequently  becomes  solid, 
and  forms  a  primitive  and,  in  some  cases,  permanent 
support  for  the  overlying  nervous  system.  In  recog- 
nition of  the  presence  of  this  cord  we  will  speak  of 
the  phylum  as  that  of  the  Chordata ;  here  are  in- 
cluded the  degenerated  Tunicates,  the  primitive  and 
somewhat  modified  Lancelet  (Amphioxus),  and  the 
great  group  of  fishes,  reptiles,  birds,  and  mammals  in 
which  a  vertebral  column,  more  or  less  well  de- 
veloped, encloses  and  protects  the  spinal  cord ;  these 
are  the  true  Vertebrata  (Balfour). 

It  is  a  matter  of  little  importance  which  of  these 
phyla  is  first  considered  in  greater  detail,  but,  as  the 
most  aberrant  are  the  Echinodermata,  it  is,  perhaps, 
convenient  to  dispose  of  them  first  of  all. 


One  of  the  best  known  types  of  the  Echino. 
dermata  is  presented  to  us  by  the  starfish  (Asterias), 
in  which  no  bilateral  symmetry  is  at  first  apparent  in 
the  adult,  though  it  is  quite  well  marked  in  the  larva. 
There  is  a  central  rounded  disc  from  which  are 
given  off  five  rays  or  "  arms  ; "  in  other  words,  we 
have  the  bilateral  symmetry  overshadowed  by  an 
acquired  radial  symmetry  (Fig.  21).  On  the  prin- 
ciples on  which  we  have  already  worked,  this  mode 
of  symmetry  in  a  freely  moving  animal  is  not,  at  once, 


Chap.  TIL]  ECHINODERMS.  59 

explicable.  To  understand  it  we  must  make  use  of 
the  method  of  comparison,  and  appeal  to  palaeonto- 
logical  evidence.  When  we  do  this  we  find  that  the 
oldest  forms  were,  like  the  still  extant  Pentacrinus, 


Fig.  21. — Astropecten  irregularis. 
7w,Madreporite. 

fixed  on  a  stalk  (Fig.  22) ;  in  other  words,  the  ances- 
tors of  the  Crinoids  being  fixed  forms  had  to  develop 
their  organs  in  different  directions  around  a  common 
centre,  so  that,  from  whatever  point  prey  or  enemy 
approached  them,  they  would  be  prepared  for  and 
ready  to  meet  them. 

In  the  great  majority  of  this  group  we  observe 
for  the  first  time  among  the  coelomate  Metazoa  a 
hard  supporting  structure  to  which  we  can  apply  the 


60    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


term   skeleton";    this  skeleton    consists   of   a    large 
number   of    firm    calcareous    plates    closely    soldered 

together.  Within,  or  just 
outside,  these  plates  there 
runs  down  every  arm,  or 
branch  of  an  arm,  a  canal 
which  contains  water,  and 
from  this  canal  there  are 
given  off  more  or  less  deli- 
cate tubes  (the  so-called 
til  toe-feet)  which  are 
connected  with  the  canal ; 


Fig.  22.— Pentaorinus  Wyville-fhomsoni.     (After  Wyville  Thomson.) 

all  the  canals  communicate  with  one  another  by 
means  of  a  ring  which  surrounds  the  mouth.  Owing 
to  the  appearance  presented  by  a  dried  starfish  the 
earlier  naturalists  spoke  of  the  areas  in  which  these 


Chap.  III.] 


ECHINODER  MS. 


61 


tube-feet  were  placed  as  the  "walks"  or  ambulacra, 
and  we  may,  therefore,  speak  of  the  ossicles  or  plates 
which  specially  support  and  protect  the  tube-feet  as 
the  ambulacra!  plates  or  ossicles.  Accom- 
panying the  radial  water-vessel  is  a  nerve-trunk  and 


an 


Fig.  23. — Diagram  of  a  Cross-section  of  an  Arm  of  a  Common  Starfish 
(Asterias  rvbens). 

On  the  left  side  the  section  is  supposed  to  pass  between  two  of  the  ambulacra! 
ossicles,  but  on  the  right  side  through  one  of  them  (ao) ;  ag,  ambulacra! 
groove ;  n,  radial  nerve ;  b,  radial  blood-vessel  ;  w,  radial  water-vessel ;  a, 
ampullae  ;  t,  tentacles  or  suckers;  ap,  adambulacral  plates  ;  sp,  spines  ;  pax, 
paxillie,  arising  from  limestone  plates ;  or,  ovary  ;  gp,  genital  jpore ;  gv, 
genital  blood-vessel ;  br,  respiratory  processes  ;  pc,  caeca  of  the  intestine. 
(After  P.  H.  Carpenter.) 

a  blood-vessel ;  while  in  the  arm  of  the  starfish  we 
find  also  generative  sacs,  and  processes  of  the  digestive 
tract ;  all  of  which  enter,  like  the  water-system,  into 
the  cavity  of  the  disc. 

If,  therefore,  we  make  a  transverse  section  (Fig. 
23)  throughout  the  arm  of  a  starfish  at  a  short 
distance  from  the  disc  we  should  cut  through  digestive, 
circulatory,  ambulatory,  generative  and  nervous 


62    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Fig.  24.— Pentacrmoid   Larva   of 
Antedon. 

A,  Quite  young  larva,  before  the  open- 
ing of  the  cup,  and  the  appearance 
of  the  radial  plates;  B,  Nearly 
mature  :  6,  basals;  o,  orals;  r,  first 
radial  s.  (After  Carpenter.) 


organs,  or  should,  in  other 
words,  have  before  our 
eyes  representatives  of  all 
the  more  important  sys- 
tems of  organs  in  the  body. 
It  is  this  phenomenon 
which  has  led  to  the 
theory  once  held  by  Cuvier, 
re-presented  by  Duvernoy, 
and,  in  our  times,  sup- 
ported with  much  vigour 
by  Haeckel,  that  the  Echi- 
noderm  is  a  colony  of 
bilaterally  symmetrical 
metazoic  animals  which 
have  become  connected  to- 
gether by  their  anterior 
ends.  It  is  more  in 
accordance  with  the  facts, 
as  at  present  known  to 
us,  to  suppose  rather  that 
the  radiate  form  has  been 
brought  about  by  a  return 
to  a  fixed  habit,  and  that 
this  mode  of  symmetry 
has  been  retained  by  in- 
heritance. In  those  forms 
which  stand  farthest  from 
the  Crinoids  the  radial  is 
again  obscured  by  a 
secondarily  acquired  bila- 
teral symmetry  (Spatan- 
gus,  Synapta)  ;  a  close  in- 
vestigation into  the  char- 
acters of  most  members  of 
the  phylum  enables  us 
to  distinguish  a  plane 


Chap.  III.]  ECHINODERMS.  63 

which    exactly    divides    the    body    into    two    similar 
halves. 

The  Echinoderniata  are  sharply  divisible  into 
two  grades ;  in  the  lower  of  these  the  animal  is  either 
fixed  by  a  stalk  throughout  life,  or,  as  in  the  case  of 
the  Rosy  Feather  star  (Antedon  rosacea)  of  our  own 
shores,  the  larva  is  fixed  by  a  stalk  (Fig.  24).  This 
grade  may  be  called  that  of  the  Pelmatozoa  ;  to 
it  belongs  the  order  of  the  Crinoidea,  with  others  now 
extinct ;  representatives  of  it  are  Rhizocrinus,  Pen- 
tacrinus,  and  Antedon. 

In  the  organisation  of  these  forms  attention  should 
be  directed  to  the  presence  of  the  cuplike  central 
portion ;  this  calyx  consists  essentially  of  a  central 
plate  and  two  sets  of  alternating  plates  five  in  num- 
ber ;  these  are  the  foasals  and  the  radials. 

In  the  higher  grade  of  the  Echinodermata,  the 
Echinozoa,  these  plates  are  often  obscured.  In  the 
regular  Sea-Urchins  (Echinoidea)  the  two  sets  of 
five  plates  can  always  be  made  out,  but  the  central 
plate  is  excavated  to  make  room  for  the  anus ;  five 
of  the  plates  become  perforated  by  the  genital  ducts 
(basals),  while  the  other  five  (radials)  are  similarly 
perforated  by  the  ocular  tentacles.  Cidaris,  Echinus, 
Echinometra  are  examples  of  the  regular  Echinoidea  ; 
by  Clypeaster  and  the  flattened  Laganum  we  pass  to 
the  edentulous  Spatangidse,  where  a  secondary  bi- 
lateral symmetry  becomes  very  apparent. 

In  the  true  starfishes  (Asteroidea),  of  which 
Asterias,  Linckia,  Oreaster,  and  Astropecten  are 
examples,  and  in  the  Ophiuroidea,  of  which  Ophiura, 
Ophiocoma,  and  Ophiothrix  are  representatives,  the 
calycinal  plates  are  often  obscured,  and  the  ambulacral 
suckers  are  limited  to  the  lower  surface  of  the  body 
and  do  not  extend,  as  in  Echinus,  from  mouth  to 
apex ;  in  the  latter  the  ambulacra  are  covered  in  by 
a  ventral  plate,  and  in  one  division  (that  of  the 


64    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Astrophytidse)  the  arms  become  more  or  less 
branched.  Lastly  we  have  the  class  of  the  Holo- 
thuroidea,  which  are  more  nearly  allied  to  the 
Echinoids  than  to  the  Asteroids ;  in  these  all  signs  of 
the  calycinal  system  have  disappeared,  the  calcareous 
skeleton  is  greatly  reduced,  and  often  consists  merely 
of  scattered  and  minute  calcareous  plates,  which  are 
sometimes  altogether  absent.  In  many  cases  the 
tube-feet  cease  to  be  arranged  in  five  regular  rows, 
and  may,  as  for  example  in  Synapta,  disappear  alto- 
gether ;  when  this  happens  there  remains  no  external 
Character  which  speaks  to  the  five-rayed  ancestry  of 
these  extreme  forms ;  in  other  words,  here  again 
external  bilateral  symmetry  is  re-acquired.  Holo- 
thuria,  Cucumaria,  Synapta,  are  the  best  known 
examples  of  this  group. 

It  is  impossible  to  escape  from  the  belief  that  the 
Artliropodsi  are  more  nearly  allied  to  the  Annulata 
than  to  any  other  group  of  the  worms,  but  they  are 


Fig.  25.— Peripatus  capensis. 

Showing  the  elongated  bilaterally  symmetrical  body,  with  the  ringed  antenna?, 
and  the  incompletely  jointed  paired  appendages  with  a  pair  of  terminal 
claws. 

sharply  distinguished  from  them  by  the  fact  that,  in 
all  cases,  one  or  more  of  the  appendages  of  the  body 
are  converted  into  organs  which  may  be  called  mouth- 
organs,  jaws,  or  gnathites.  Some  idea  of  the  primi- 
tive form  may  be  gathered  from  Peripatus,  which 
is  the  simplest  Arthropod  known  to  us.  The  body 
was  elongated,  distinctly  bilaterally  symmetrical,  the 
praestomium  was  provided  with  tactile  antennae,  and 


chap,  in.]  ARTHROPODA.  65 

at  the  sides  of  the  body  there  were  a  number  of 
appendages  which  were  only  incompletely  ringed,  but 
the  presence  of  which  afforded  evidence  of  metameric 
segmentation.  The  mouth  was  near,  though  not 
quite  at,  the  anterior  end  of  the  body,  and  at  its  side 
were  a  pair  of  slightly  modified  appendages ;  the 
anus  was  posterior  and  terminal.  The  excretory 
organs  were  on  the  type  of  the  Annulata,  and  were 
arranged  metamerically.  Peripatus  may  form  the 
type  of  the  Protracneata. 

In  all  the  remaining  Arthropoda,  some  of  which 
in  all  probability  did  not  have  a  Peripatus-like  an- 
cestor, but  have  acquired  a  form  similar  to  that  of  the 
descendants  of  such  an  ancestor,  owing  primarily  to 
similar  external  conditions  and  similar  necessities  of 
life  (homoplasy,  see  page  12),  the  appendages  are  dis- 
tinctly jointed,  so  that  the  separate  parts  can  be 
moved  on  one  another ;  the  mouth  is  often  some  way 
from  the  anterior  end,  and  excretory  organs  of  the 
annulate  type  are  never  found. 

In  the  simpler  forms  the  greater  number  of  meta- 
meres  remain  distinct,  but  in  all  divisions  there  is  a 
marked  tendency  for  the  metameres  at  the  anterior  end 
to  fuse  into  a  head,  and  in  some  cases  also  into 
a  thoracic  region. 

They  are  divisible  into  three  great  groups  :  A. 
Crustacea,  B.  Arachnida,  C.  Tracheata.  In 
all  three  chitin  is  largely  developed  in  the  integument ; 
and  they  are  all,  in  addition,  remarkable  for  the  total 
absence  of  those  delicate  protoplasmic  processes  which 
we  have  learnt  to  know  as  cilia. 

A.  The  great  majority  of  the  Crustacea  are 
aquatic  forms,  and  they  either  breathe  the  oxygen  dis- 
solved in  the  water  in  a  vague  manner  (that  is  to  say, 
no  special  respiratory  organs  are  developed,  and  the 
exchange  of  gases  is  effected  through  the  walls  of  the 
body),  or  they  are  provided  with  outgrowths  of  the 

F— 16 


66    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

body  wall,  which  are  known  as  gills  or  branchiae ; 

the  presence  of  these  has  caused  the  name  of 
Branchiata  to  be  given  to  this  division  of  the  Arthro- 
poda.  The  greater  number  of  the  segments  carry  a 
pair  of  appendages,  and  the  great  majority  of  these 
are,  in  the  lower  forms,  exactly  similar  in  character 
(Fig.  26,  5a).  The  metameres  remain  separate,  and  are 


Fig.  26. — Various  Branchiopoda. 

1,  Nelmlia  bipes  (shell  removed  on  one  side);  2,  Estlieria  sp. ;  8nt 'dorsal;  S6, 
ventral  aspect  of  Lepidurus  angassi  ;  4,  larva  of  Apus  cancifornus  ;  5«,  adult 
female  of  Branchipus  stagnalis  ;  5b,  5c,  larvif  ;  6,  larva  of  Artemia  saliua. 

often  very  numerous  ;  in  the  higher  forms  they  tend, 
in  a  most  remarkable  manner,  to  be  limited  to  about 
twenty,  and  the  dorsal  parts  of  the  hard  exoskeleton 
become  fused  in  the  anterior  region  (Fig.  27). 

In  all,  the  mouth  is  moved  so  far  back  from  the 
anterior  end  of  the  body  that  two  pairs  of  appendages 
(antennae)  lie  in  front  of  it. 

They  are  divisible  into  the  Eiitomostraca,  so 
called  from  the  slight  amount  of  fusion  of  the 


chap,  in.]  CRUSTACEA.  67 

exoskeleton  of  the  separate  metameres,  and  the 
Malacostraca,  which  were  so  called  because  their 
covering  is  soft  as  compared  with  the  hard  shell  of 
the  oyster  or  the  snail.  In  both  divisions  we  find 
members  which  .have  become  parasitic  in  habit,  and 


Fig.  27. — The  Common  Prawn  (Palcemon  serratus). 

in  which,  consequently  the  characteristics  of  ar- 
thropod organisation  are  more  or  less  modified  and 
obscured. 

In  the  Entomostraca  we  never  have  more  than 
three  pairs  of  appendages  converted  into  Onatliites, 
or  jaws;  the  appendages  behind  the  genital  orifices 
never  carry  appendages  (Fig.  26;  5«),  and  the  young 
nearly  always  make  their  appearance  as  unsegmented 
larvae  with  two  or  three  pairs  of  appendages,  of 
which  two  are  constantly  biramose  (Naiiplius 
larvae)  (Fig.  26  :  4,  56,  6). 

1.    The  Brancliiopoda  have,    as    their    name 


68    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

implies,  the  function  of  respiration  undertaken  by  some 
of  the  appendages  ;  the  body  is  often  provided  with  a 
fold  (Fig.  28  ;  1,  2),  which  is  derived  from  the  dorsal 
portions  of  the  anterior  metameres,  and  forms  a  back- 
wardly-directed  free  carapace.  Such  are  Apus  and 
Daphnia  ;  Nebalia  forms  a  link  of  connection  with  the 
Malacostraca. 


1,  Daphnia   pulex  ; 


Fig.  28. — Various  Entomostraca. 


;  2,  Candona  hispida;  3a,  adult  female  of  Cyclops  quartri- 
pornit;  6,  c,  d,  larv;«;  4,  Cetochilus  septentrionalis  ;  5,  Sappbirina  ovato 
lanceolata  ;  6.  Nicotboe  astaci  (parasitic  on  tbe  gills  of  the  lobster)  ;  7,  Nau- 
plius  stage  of  copepod.  (From  Woodward.) 

2.  The  Copepoda  have  a  small  stout  body, 
covered  by  a  carapace  ;  one  pair  of  the  antennae  are 
large  and  oar-like  (Fig.  28  ;  3a),  and  retain  the  primi- 
tive locomotor  function  that  they  had  in  the 
nauplius  stage.  Cyclops  and  Cetochilus  are  free- 
swimming  forms  ;  some,  like  Sapphirina  (Fig.  28  ;  5) 
are  temporary  parasites  ;  others,  like  Nicothoe(Fig.  28; 
6),  which  lives  on  lobsters  and  crayfishes;  Dichelestium, 
which  is  found  on  the  sturgeon,  and  Lernsea,  which  lives 
on  the  cod  and  other  fishes,  are  still  more  modified  ; 


Chap.  III.] 


CRUSTACEA. 


69 


while  the  extreme  modification  is  seen  in  Argulus,  a 
common  parasite  on  the  stickleback. 

3.  In  the  Ostracoda  the  carapace  forms  a  com- 
pletely   bivalve   shelly    covering   for    the   body,  the 
abdominal  region  of  which  is  greatly  reduced.    Cypris 
arid  Cythere  are  examples. 

4.  Although   the  Cirripedia   are,  when   adult, 
greatly  altered  by  their  fixed  or  parasitic  habit,  they 
leave  the  egg  as  Naupliiform  larvae  ;  these  become 
attached  by  their  anterior 

ends,  and  enclosed  in  a 
sac-like  mantle  formed  by 
the  integument ;  this 
either  remains  soft,  as  in 
Alcippe,  which  lives  in 
cavities,  and  is  thereby 
protected,  or  undergoes 
calcification,  when  a 
greater  or  less  number  of 
plates  become  developed. 
The  anterior  region  is 
either  broad,  as  in  the 
acorn  shell  (Balanus),  or 
drawn  out  into  a  stalk,  as 
in  the  barnacle  (Lepas). 

5.  The      Ceiitrogo- 
iiida,    or,    as   they    are 
often      called,      Rliizo- 
cepliala,     are     usually 
found  on   the   bodies    of 
higher  Crustacea  after  the 
nauplius  stage  is  passed. 
They    are    endoparasitic, 

and,  later  on,  form  a  sac  without  limbs  on  the  outer 
surface  of  their  host's  body.  To  this  group  belong 
Peltogaster  and  Sacculina. 

B.  The  Malacostraca  have  almost  constantly 


Fig.  29.— Squilla  mantis. 


yo    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


twenty  segments  to  their  body,  and  all  but  one  of 
these  bear  appendages ;  as  many  as  six  may  be  con- 
verted into 
gnathites,  and 
the  larvae  ordi- 
narily, though 
not  always,  are 
set  free  at  a 
later  than  the 
nauplius  stage. 

l.The  Pod- 
optitlialmata 
are  so  called 
from  the  fact 
that  their  eyes 
are  placed  on 
stalks  (Fig.  29); 
in  them  some  of 
the  dorsal  por- 
tions of  the 
thoracic  m  eta- 
meres  take  part 
in  the  formation 
of  a  carapace. 
Such  are  cray 
fishes,  lobsters, 
shrimps,  and 
crabs. 

2.  The  He- 
d  ra  op  lit  li  al- 
ma ta  have  the 
eyes  sessile,  and 
no  carapace  is 
developed ;  the 
A  in  phi  pod  a  (e.g.  sandhopper)  are  the  least  modi- 
fied ;  some  of  the  Isopoda  (such  as  the  wood-louse) 
are  fitted  to  and  do  dwell  on  land,  while  the 


Fig.  30.  -Limulus  moluccanus. 


Chap.  III.] 


Fig.  31.— Scorpio  occitanus. 


72    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY 

Laemodipoda  are  modified  by  parasitism,  and  have 
the  abdominal  region  rudimentary  (e.g.  Cyamus,  which 
is  found  on  the  skin  of  whales). 


Fig.  32.—  Ammothoa  pycnogonoides. 

B.  The  Arachnida  are  arthropods,  in  which  the 
mouth  is  never  placed  so  far  back  that  any  of  the 
appendages  become  antennary  organs ;  the  second  and 
succeeding  four  (at  most)  pairs  of  appendages  have 
their  basal  portions  ranged  round  the  mouth,  the 
functions  of  which  these  parts  subserve.  The  free 
portions  of  the  six  anterior  appendages  take  on  various 


Chap.  III.] 


ARACHNIDA 


73 


duties.  Respiration  is  effected  by  flattened  processes 
attached  to  the  appendages  behind  the  generative 
pores  (which  are 
always  placed  com- 
paratively far  for- 
wards), and  they 
either  carry  blood 
or  contain  air,  or 
disappear  and  are 
replaced  by  tracheae. 
The  hinder  part  of 
the  body  never 
carries  jointed  ap- 
pendages. 

1.  Haemato- 
branctiiata. — 
These      are      to-day 
represented    by    the 
king-crab  (Limulus  ; 
Fig.    30).     In   them 
the    respiratory     la- 
mellse  contain  blood, 
and  the  hinder  por- 
tion of  the   body  is 
fused    into   a    single 
mass,  while  the  ter- 
minal   spine    is    of 
great  length. 

2.  jErobran- 
ctiiata. — Such    are 
the    scorpion    (Scor- 
pio;  Fig.  31)  and  the 
spiders  (Mygale).    In 

flipop  fhp  rpcnriratrvrv       A»  Female,  nat.  size;    B.   male,  nat.  size ;  o, 

inese  T-ne  respiratory       head  ()f  lualei  eniarged. 

lamellae  are  sunk  into 

depressions  of  the  body,  and  contain  air  (the  so-called 

lungs   or   lung-books).       The  hinder   portion   of   the 


Fig.  33. — Pentastomum  tcenioides. 


74    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

body  is  either  elongated  and  distinctly  jointed,  with  a 
short  terminal  spine,  or  is  greatly  contracted,  and 
globose  in  form. 

3.  Upoforanchiata. — In  these  the  respiratory 
lamellae  are  lost,  and  if  any  special  respiratory  organs 
are  developed,  they  are  in  the  form  of  tracheal  tubes. 
Here  belong  the  Acarina  (mites,  ticks),  with  an 
nnsegmented  abdomen,  and  often  a  sucking  mouth; 
the  Pedipalpi  (Harvestmen),  with  a  segmented 
abdomen;  and  the  Pyciiogonida  (no-body  crabs), 
in  which  prolongations  from  the  gastric  cavity  extend 
into  the  enormously  long  legs  (Fig.  32). 

Appended  to  this  group,  but  considerably  altered 
by  parasitism,  so  that  when  adult  they  have  elongated 
worm-like  bodies,  with  but  two  pairs  of  mouth  hooks 
to  represent  the  appendages,  are  the  Peiitastomida, 
the  best  known  example  of  which  is  the  Pentastomum 
tsenoides,  which  is  found  in  the  frontal  sinuses  of 
dogs'  skulls  (Fig.  33,  A,  B,  c). 

C.  The  third  division  of  Arthropoda  is  that  of  the 
Tracheata ;  in  them  there  is  always  one  pair  of 
antennae  in  front  of  the  mouth,  the  gnathites  may  be 
very  profoundly  modified  ;  respiration  is  effected  by 
means  of  air  tubes  (tracheae),  which  are  regularly 
arranged  and  richly  developed  within  the  body.  They 
are  divisible  into  a  lower  and  a  higher  group,  of  which 
the  former  has  comparatively  few  representatives  ; 
the  other  more  than  all  the  rest  of  the  animal 
kingdom. 

I.  Myriopoda  or  Centipedes  and  Millipedes. — 
In   these   most  of  the  metameres   are    separate   and 
distinct,  or  are  united  by  pairs,  and  all  are  provided 
with  a  pair  of  jointed  appendages.     The  mouth  organs 
are  not  greatly  modified  ;  they  are  all  terrestrial. 

II.  Hcxapoda  or  Insects. — In  the  vast  assem- 
blage   of    forms    associated    under    this    head,    the 
appendages    of    the    adult    are     never    functionally 


Chap.  III.] 


INSECT A. 


75 


developed  behind  the  region  of  the  thorax ;  one  pair 
of  appendages  form  the  prse-oral  antennae,  and  the 
metameres  do  not  exceed  twenty  in  number.  They 
are  sharply  divisible 
into  two  great  sub- 
divisions, according  as 
they  are  or  are  not 
provided  with  wings ; 
with  the  latter,  of 
course,  we  must  as- 
sociate those  in  which 
wings  are  found  in 
one  sex  only,  or  are 
rudimentary,  or  of 
whose  ancestral  ex- 
istence (as  in  the  case 
of  parasites),  we  have 
sufficient  evidence. 

A.  Aptera,     or 
true    wingless    forms 
such    as  the    spring- 
tails     (Podura),     and 
bristle-tails  (Lepisma). 
In    the     simplest    of 
these   the   mouth  or- 
gans can  work  either 
from  side  to  side,   or 
from     before      back- 
wards ;    the  tracheae, 
however    well    de- 
veloped, and  they  are 
often  only  poorly  so, 

never  anastomose  with  one  another  (Fig.  34). 

B.  Pterygota. — Here   belong   all  the  remaining 
insects,   which    are    either  winged,  that   is,  provided 
with  two  pairs  of  membranous  dorsal  outgrowths  in 
the  region  of  the  thorax,   which  can  be  moved  by 


Fig.  34. — Orchcsella  cincta,  enlarged. 


7 6    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

muscles  and  serve  for  flight;  or  one  pair  only  is 
developed,  or  is  developed  in  one  sex  only,  or  both 
pairs  are  more  or  less  rudimentary.  The  organs  of 
the  mouth  are  adapted  for  biting  and  cutting,  or  for 
sucking,  and  the  abdominal  metameres  are  often  more 


Fig.  35.— Cockroaches  :  A,  Male ;  u,  Female ;  c,  Young. 

or  less  reduced  ;  the  generative  pores  are  placed  far 
back,  and  respiration  is  always  effected  by  tracheal 
tubes  or  modifications  thereof. 

a.  Mandibulata.— In  this  series  the  mouth 
organs  are  adapted  for  cutting  and  biting,  and  move 
from  side  to  side;  or  are  converted  into  licking 
organs. 

1.  Ortlioptera  :  cockroaches,  grasshoppers,  and 
locusts. — With  the  anterior  pair  of  wings  converted 


chap,  in.]  INSECTA.  77 

into  wing  covers,  and  the  posterior  often  functional 
in  the  males  only.  No  true  metamorphosis  (Fig.  35). 

2.  Neuroptera:    dragon-flies,     termites, — With 
two  pairs  of  membranous  wings.     A  true  metamor- 
phosis, or  the  life  history  consisting  of  three  periods, 
an  active  larval,  a  quiescent  pupal,  and  an  active 
perfect  or  imaginal  condition. 

With  this  group  may  be  placed  the  Triclioptera 
(caddis-flies). 

3.  Coleoptera;  beetles,  cockchafers,  lady-birds. 
—  Anterior  pairs  of  wings  converted  into  wing-covers ; 
these  are  distinctly  horny.     True  metamorphosis. 

The  parasitic  Strepsiptera  come  nearest  to  this 
order. 

4.  The    Hymenoptera    (bees,    ants)     have   the 
mouth    organs   adapted    for  licking,    as   well   as    for 
biting  and  cutting.     Both  pairs  of  wings  functional. 
Metamorphosis  complete. 

£.  Haustellata. — In  this  series  the  mouth 
organs  move  from  before  backwards,  or  serve  as 
stabbing  or  sucking  organs. 

5.  Hemiptera    (bugs,    aphides,    lice).  —  Mouth- 
organs  stabbing  and  sucking.     Anterior  pair  of  wings 
functionless ;  in  parasites  both  may  be  rudimentary. 
Metamorphosis  generally  incomplete. 

6.  Diptera  (flies,  fleas). — Mouth  organs  stabbing 
and  sucking ;  anterior  wings  functional,  the  posterior 
possibly  represented  by  the  small  knobbed  "  balancers  " 
(halteres).     Metamorphosis  complete. 

7.  L-epidoptera     (butterflies,     moths). — Mouth 
organs  form  a  sucking  apparatus,  with  no  power  of 
stabbing ;  both  pairs  of  wings  functional.     Metamor- 
phosis complete. 


The  Mollusca  form  a  well-marked  phylum,  the 
essential  characters  of  which  would  be  represented  in 


78    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

some  such  schematic  Mollusc  (Lankester)  as  that  here 
figured.  The  oblong  body  is  bilaterally  symmetrical, 
and  the  prsestomium,  as  in  Peripatus,  is  provided 
with  a  pair  of  tentacles  (Fig.  36,  A,  a)  •  the  mouth 
(B,  o)  is  on  the  lower  surface,  and  near,  though  not 
at  the  front  end,  while  the  anus  (m)  is  median,  dorsal, 

*  I 


d 


Fig.  36.— Diagrams  of  the  Typical  Structure  of  a  Mollusc.  A,  from 
above ;  B,  from  below. 

a,  Tentacles  of  head  ;  5,  Head  ;  c,  edge  of  mantle ;  e,  outline  of  foot  seen  through 
the  mantle,  which  is  supposed  to  he  transparent;  /,  edge  of  shell-follicle; 
g,  shell ;  h,  osphradiiiiu  (Sprcngel's  olfactory  organ);  i,  ctenidia  (gills);  fc. 
generative  orifice  (paired);  I,  aperture  of  one  of  the  nephrldia  (excretory 
organs) ;  m,  anus ;  n,  foot  where  it  extends  beyond  the  visceral  mass ;  o, 
mouth ;  p,  plantar  surface  of  foot.  (After  Ray  Lankoster.) 

and  posterior ;  right  and  left  of  this  anal  opening  we 
find  the  orifices  of  the  excretory  organs  (I),  and  near 
them  those  of  the  genital  ducts  (k). 

So  far  the  creature  presents  no  characters  other 
than  such  as  we  might  expect  to  find  in  any  ccelomate 
Metazoon  ;  in  addition,  there  are  four  characters  of 
greater  significance.  The  ventral  surface  is  produced 
into  a  more  or  less  triangular  muscular  outgrowth, 
which  is  known  as  the  foot ;  the  dome-like  dorsal 
surface,  which  contains  the  chief  mass  of  the  viscera, 


Chap.  III.]  MOLLUSCA.  79 

is  protected  by  a  hard  body,  the  shell  (#),  and  this 
shell  is  derived  from  a  primary  shell-sac  (/) ;  the 
walls  on  either  side  of  the  middle  line  of  the  body  are 
produced  into  free  folds,  the  pair  of  which  make  up 
the  mantle,  and  on  either  side  of  the  body  there  are 
given  off  comb-like  processes  (ctenidia)  (i),  which 
are  ordinarily  known  as  the  gills.  Indications  of 
metameric  segmentation  are  rare,  and  are  only 
obscurely  indicated  in  the  majority  of  the  cases  where 
they  are  to  be  detected. 

The  Mollusca  may  be  primarily  divided  into  those 
in  which  the  region  of  the  head  is  reduced  or  lost, 
and  those  in  which  it  takes  on  more  special  characters. 
The  former  are  conveniently  known  as : 

A.  JLipocephala. — This  division  contains  only 
the  group  *of  the  JLamellibranchiata  or  mussels 
and  oysters.  In  these  the  primitively  single  shell  is 
divided  into  two  bilaterally  symmetrical  halves,  and 
the  two  divisions  of  the  shell  are  only  different 
(Oyster  :  Myodora)  in  size  and  character,  when  one 
side  comes  to  be  that  on  which  the  animal  ordinarily 
reposes,  or  when  it  ceases  to  live  in  an  upright 
position ;  the  foot  may,  as  in  boring  forms,  be  of  con- 
siderable size,  or  it  may  be  greatly  reduced,  as  in  the 
oyster,  which  remains  for  long  periods  at  the  same 
place. 

This  shell  is  brought  together  by  special  adductor 
muscles,  of  which  two  pairs  are  found  in  many  adults, 
and  have  been  observed  in  the  young  of  some  which 
(oysters)  have  only  one  pair  in  adult  life.  The 
ctenidia,  which  commence  as  separate  ciliated  filaments 
in  two  rows  on  either  side,  ordinarily  undergo  a  large 
amount  of  fusion  or  concrescence,  whereby  they  are 
converted  into  perforated  plate-like  structures  which 
have,  among  others,  a  respiratory  function.  In  some 
the  mantle  never  extends  beyond  the  limits  of  the 
shell,  and  these  are  the  : 


8o    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

I.  Asiphoniata,    such  as  the  mussels  and  the 
oysters. 

II.  In  others  the  mantle  is  produced  into  two  more 
or  less  elongated  siphons  (Fig.  37,  Siphoniata)  and 
these  siphons  are  either  not  retractile  as  in  the  cockle 
(Cardium),  and  the  immense  Tridacna ;  or  the  siphons 
can  be  retracted  by  special  muscles  (Sinupalliata), 
as  in  Pholas,  Solen,  and  Mactra. 

B.   In  the  higher  division  of  the  Mollusca  the 


Fig.  37. — Mya  arenaria,  a  Siphonate  Latnellibranch. 
ex,  Excurrent  ;  in,  incurrent  siphon ;  a,  anterior ;  a',  posterior  adductor  muscle; 
gg,  branchiae  ;  /,  foot ;  t,  labial  tentacles  ;  o,  mouth ;  s,  stomach  ;  d,  Intestine; 
p,  muscle  of  the  foot. 

cephalic  tentacles  and  eyes  are  retained,  and  within 
the  cavity  of  the  pharynx  there  is  developed  a  special 
rasping  organ  or  tongue,  the  presence  of  which 
justifies  the  name  Glossopliora,  which  is  applied  to 
this  series.  In  a  number  of  these  the  foot  becomes 
divided  into  three  well-marked  regions,  but  in  the 
lowest  group, 

1.  Gastropoda,  the  foot  is  ordinarily  simple,  and 
only  constricted  into  three  regions ;  it  -is  broad  and 
flattened.  In  a  large  number  the  body  undergoes  a 
twisting  round  its  central  axis,  in  consequence  of 
which  the  two  sides  of  the  body  come  to  be  unequally 


Chap.  III.] 


MOLLUSC  A. 


Si 


the 


or  asymmetrically  developed.  The  appearance  of  this 
torsion  allows  us  to  divide  the  Gastropoda  into  a 
lower  or  more  primitive,  and  a  higher  or  more 
differentiated  series. 

o.  Isopleura. — In  these  the  two  sides  of 
body  are  equally  developed,  and 
many  of  the  characters  of  the 
primitive  mollusc  are  retained 
unchanged.  Here  we  have  the 
Polypfacophora,  represented 
by  the  Chitons,  in  which  the  shell 
is  broken  up  into  eight  pieces  ar- 
ranged in  a  fashion  to  which  it  is 
difficult  to  refuse  the  name  of 
metameric  arrangement  (Fig.  38) ; 
and  the  Neomeniidse,  and  the 
Chaetodermatidae,  in  which 
the  shell  is  represented  by  spicules 
only. 

£.  In  the  Anisopleura  we 

have  an  exceedingly  interesting  phenomenon ;  while 
the  body  undergoes  torsion,  the  nerve- cords  that  run 
down  the  sides  of  the  body  may  or  may  not  be  impli- 
cated in  the  change.  Where  they  are  not  we  have 
the  Euthyneiira,  which  either,  like  Aplysia  and 
Doris,  continue  to  breathe  by  gills  the  oxygen  dis- 
solved in  water,  or  like  the  pond-snail  (Lymnceus), 
the  garden-snail  (Helix),  and  the  slug  (Limax),  have 
their  gills  aborted,  and  a  breathing  chamber  or  lung 
formed  by  the  apposition  of  part  of  the  edge  of  the 
mantle  to  the  side  of  the  body. 

In  the  Streptoneura  the  nerve-cords  are  impli- 
cated in  the  general  torsion  of  the  body,  and  form  a 
figure  of  eight  loop ;  in  the  Zygobranchiata,  of 
which  the  ear-shell  (Haliotis)  and  the  limpet  (Patella) 
are  examples,  the  right  and  left  gills  become  re- 
spectively the  left  and  right,  and  are  equal  and 
c— 16 


—  Chiton  mag- 
nificus. 


82    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

symmetrical ;  in  theAzygol>raiicliiata,such  as  Palu- 
dina,  Dolium,  the  cowry  (Cyproea),  and  the  whelk 
(Buccinum),  the  left  gill  and  excretory  organ  become 
aborted ;  some  members  of  this  division,  the  so-called 
Heteropocla,  become  modified  to  a  free-swimming 
life,  as  Atlanta  or  Firuloides. 

II.  The    ancient    group    of    the    Scaphopoda 
exhibits   some  primitive  characters,   but  is  specially 
remarkable  for  its  elongated  elephant- 
tooth-like    shell    (Dentalium)   which   is 
open  at  either  end  (Fig.  39). 

III.   The   Pteropoda   closely   ap- 
proach  in   many  important  characters 
the   next   succeeding   group,   but   they 
are   most   conveniently  kept  separated 
from  them.     The  anterior  portion  of  the 
foot  (propodium)  surrounds  the  head, 
and  the  median  part  (mesopodiuiii) 
Fig.  39.  —Shell     ig  converted  into  a  pair  of  napping  fin- 
ofDentaiinm     like  organs  by  means  of   which   these 

elephant!-  -,.        v,          .  J  ,  -,-,-, 

num.  ordinarily  minute  creatures  are  enabled 

to  swim  about  on  the  surface  of  the 

ocean.     According  as  they  have  or  have  not  a  shell, 

they  are  called  Thecosomata  (Hyalea,  Cymbulia), 

or  Oymnosomata  (Clione,  Pneumodermon). 

IV.  The  last  and  highest  division  of  the  Mollusca 
is  formed  by  the  Cephalopoda ;  the  propodium  is 
here  produced  into  a  number  of  long  tentacular  pro- 
cesses or  arms,  on  which  suckers  are  not  unfrequently 
developed  ;  the  mesopodium  of  either  side  unites  with 
its  fellow  to  form  an  incompletely  or  completely 
closed  siphonal  tube,  which  serves  as  the  chief  organ 
of  locomotion.  The  shell  is  external  or  internal, 
coiled  or  simple,  or  completely  absent. 

a.  The  ancient  group  of  the  Tetrabrancliiata,  to 
which  many  fossil  forms  belong,  is  represented  to-day 
by  a  single  genus,  Nautilus ;  they  receive  their 


Chap.  HI.]  CEPHALOPODA.  83 

name  from  the  possession  of  two  pairs  of  gills,  with 
which,  an  exceptional  circumstance  among  Molluscs, 
are  associated  two  pairs  of  ante-chambers  to  the  ven- 
tricle. The  siphon  is  incomplete,  the  propodial  ten- 
tacles are  numerous  and  devoid  of  suckers  ;  the  shell 
in  external,  chambered,  and  coiled  (Fig.  40). 

B.  The  Diforancliiata  have  either  eight  arms  as 
in  the  Octopus,  or  ten  as  in  the  squid  (Loligo),  or  Sepia  ; 


Fig.  40.— Section  of  the  Shell  of  the  Pearly  Nautilus,  showing  the  coil 

of  chambers,  and  the  animal  in  the  largest,  or  that  last  formed  (z). 
a, Mantle;  b,  dorsal  fold;  o,  shell-muscle;  ii,  Siphuncle;  k,  funnel  or  Bi phonal 
tube;  n,  hood;p,  tentacles;  s,  eye;  x,  septa  between  the  chambers. 

there  is  only  a  single  pair  of  gills  and  auricles,  and 
the  arms  are  provided  with  suckers  (Fig.  41). 

It  has  long  been  the  custom  to  divide  the  members 
of  the  Animal  Kingdom  sharply  into  the  two  great 
groups  of  "  Vertebrata  "  and  "  Invertebrata";  we  have 
seen,  however,  that  the  most  scientific  separation  is 
that  into  uni-cellular  and  multi-cellular  organisms, 
Protozoa  and  Metazoa ;  and,  next,  that  the  lower 
Metazoa  have  no  signs  of  that  body-cavity  or  ccelom 
which  becomes  so  well  marked  a  part  of  the  organisa- 
tion of  the  higher  forms ;  and,  lastly,  we  have  seen 
that  the  Echinodermata,  the  Arthropoda,  and  the 


84    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Mollusca  form  three  very  distinct  branches  or  phyla, 
the  common  ancestor  of  which  is  to  be  sought  for 
only  in  a  simple  worm.  Of  equal  value  with 
these  is  another  phylum,  which  may  be  most  conve- 
niently spoken  of  as  that  of  the  Chorda ta,  distin- 
guished from  the  rest  by  the  association  of  two 
characters,  the  temporary  or  permanent  possession 
of  a  rod  underlying  the  central 
dorsally-placed  nervous  system, 
and  the  similarly  temporary  or 
permanent  possession  of  clefts 
or  passages  at  the  sides  of  the 
head  and  neck,  which  open  to 
the  exterior  (visceral  clefts). 
Either  one  of  these  cha- 
racters may  be  seen  in  certain 
members  of  that  heterogeneous 
mob,  which,  partly  from  the 
nature  of  things,  and  partly 
from  the  imperfect  condition  of 
our  knowledge  respecting  them, 
must  be  retained  in  the  group 
of  Vermes. 

Among,  or  standing  near  to, 
the  Platyhelminthes,  are  some 
elongated,  free-swimming,  ma- 
rine forms  which  are  known  as  the  Nemertinea. 
These  worms  are  provided  with  a  dorsal  proboscis,  which 
is  enclosed  in  a  sheath.  The  relations  of  this  proboscis 
to  its  sheath  are  shown  in  Fig.  42  A,  while  Fig.  42  B  ex- 
hibits in  diagramatic  form  the  relation  of  certain  parts 
in  one  of  the  lowest  of  fishes  (the  lamprey) ;  a  comparison 
of  the  relations  of  these  structures  (proboscis  and  its 
sheath  on  the  one  hand,  and  chorda  dorsalis  on  the 
other)  with  (a)  the  dorsal  surface  of  the  body  and  (£) 
the  digestive  tract,  reveals  very  striking  resemblances, 
which  come  to  be  of  still  greater  significance  when  we 


41.  —  The    Common 
Cuttlefish. 


Chap.  III.] 


ENTEROPNE  us  TI. 


combine  with  them  the  knowledge  of  the  fact  that,  in 
certain  Nemertines,  the  nerve  cords,  instead  of  lying 
at  the  sides  of  the  body,  tend  to  take  up  a  dorsal  posi- 
tion. Whether  or  no  Hubrecht  is  right  in  regarding 
the  Nemertinea  as  giving  us  indications  of  where  to 
look  for  the  ancestral  form  of  the  Chordata,  it  is 
clear  that  we  must  sharply  distinguish  them  from  the 
group  of  the  Platyhelminthes,  with  which  they  have 


Fig.  42. — A,  Diagram  to  show  the  relation  of  the  proboscis  (pbs)  to  the 
surface  of  the  body  and  to  the  sheath  of  the  proboscis  (pbs) ,  in  the 
Nemertinea;  (B)  diagram  of  Petromyzon  (the  lamprey)  showing  the 
hypophysis  cerebri  (hyp) ;  the  chorda  dorsalis  (ch) j  the  mouth  (m) ; 
and  the  anus  (a).  (After  A.  A.  W.  Hubrecht.) 


been  hitherto  very  closely  associated.     Lineus,  Cari- 
nella,  Polia,  are  examples  of  this  group. 

So,  again,  in  another  group  of  "  worms,"  the 
Enteropncusti,  the  sole  representative  of  which  is 
the  remarkable  Balanoglossus  (Fig.  43),  the  anterior 
portion  of  the  enteron  divides  into  a  ventral  and  a 
dorsal  portion  ;  the  former  retains  its  nutrient  office, 
but  the  latter  has  chitinous  lamellae  developed  in  its 
walls ;  between  these  clefts  (br)  appear,  which  finally 
open  on  the  surface  of  the  body  ;  blood-vessels  are  richly 
distributed  to  the  walls  of  the  arches,  and  the  water 
taken  in  by  the  mouth  passes  through  the  clefts  to 
the  exterior.  In  Balanoglossus,  therefore,  just  as 


86    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

much  as  in  a  fish,  we  have  gills    developed    at    the 

sides  of  the  anterior  region  of  the  digestive  tract. 

With  regard  to  the  Chordata,  however,  it  is  to  be 

distinctly    borne    in    mind  that  l>oth  these    organ-; 

(notochord  and  gill-slits)  are  to  be  found,  and  we  may, 

therefore,  look 
for  the  ancestral 
or  ideal  Chordate 
in  an  elongated, 
bilaterally  sym- 
metrical, meta- 
merically  seg- 
mented animal, 
in  which  the  cen- 
tral nervous  sys- 
tem, dorsal  in 
position,  was 
supported  by  a 
rod  of  firm  tis- 
sue, in  which  the 
sides  of  the  body 
and  pharynx 
were  perforated 
by  gill  slits  ;  and 
in  which  the 
mouth  was 

Fig.  43.—  Young  Balanoglostus  seen  from  the  ride.   P^ced      on      the 
br.  Branchial  slita;  x!2.  (After  Pagenstecher.)  ventral     surface, 

not  far  from  the 

front  end  of  the  body.  The  Chordata  fall  into 
three  well-marked  groups ;  in  one  degeneration  has 
proceeded  to  an  extent  so  considerable,  that  in  many 
all  indications  of  a  chordate  ancestor  are  completely 
lost;  these  are  the  Urochordata  or  so-called 
Tunicata.  In  another,  many  primitive  characters, 
such  as  the  original  segmentation  and  the  notochord, 
are  retained  unchanged,  but  in  some  few  points 


chap,  in.]  CHORD  ATA.  87 

there  would  seem  to  be  degradation;  these  are  the 
Ccphalochordata ;  and,  lastly,  we  have  the  true 
Verteorata  or  Craniata. 

A.  Cephalochordata. — Of  these  the  only  exam- 
ple is  the  Lancelet  or  Amphioxus,  in  which  the  noto- 
chord,  pointed  at  either  extremity,  extends  from  one 
end  of  the  body  to  the  other ;  the  number  of  gill  slits 
is  very  great,  and  they  are  covered  over  by  an  out- 
growth of  the  body  wall  which  grows  down  on  either 
side,  and  unites  along  the  ventral  line,  leaving  a  pore 
for  the  exit  of  the  water  (atrial  pore).    The  original 
segmentation  of  the  muscles  of  the  body  is  not  ob- 
scured ;  the  mouth  is  over-hung  by  a  projecting  hood, 
and  furnished  with  a  number  of  tentacles  (cirri) ;  the 
liver  is  represented  by  a  very  slight,  blindly  ending 
outgrowth  of  the  enteric  tube,  and  renal  organs  are  very 
obscurely  indicated ;  there  is  no  centralised  heart,  and 
appendages  are  completely  wanting.     The  eye  is  only  a 
pigment  spot,  and  no  signs  of  an  ear  have  been  detected. 

B.  Urochordata. — In  no  division  of  the  animal 
kingdom  has  the  value  of  the  study  of  development 
been   of   more  -importance  than    in  this,   for  it   has 
revealed  the  presence  of  a  notochord,  and  the  essen- 
tial resemblance  between  their  gill  clefts  and  those  of 
the  Cephalochordata  ;  while  in  none  has  the  applica- 
tion of  the  principle  of  degeneration  (Dohrn ;  Lan- 
kester)  been  more  instructive. 

In  but  few  forms  is  the  notochord  retained 
throughout  life,  and  in  these  it  is  found  in  the  tail 
only,  Perennichordata  (e.g.  Appendicularia) ;  in 
the  rest,  Caducichordata,  the  caudal  notochord  is 
present  in  the  larva  only,  or  is  never  developed  at  all ; 
in  these,  just  as  in  Amphioxus,  outgrowths  of  the  body 
wall  enclose  the  true  sides  of  the  body,  and  give  rise 
to  an  atrial  chamber,  by  whose  pore  the  water  of  re- 
spiration, and  often  also  the  waste  matters  of  digestion 
finally  make  their  way  to  the  exterior  (Fig.  44). 


88    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Some  of  the  Caducichordata  remain  solitary 
throughout  life,  e.g.  Ascidia,  or  Boltenia  (which  is 
remarkable  for  its  long  stalk)  ;  others  become  fused 
into  a  common  colony,  as  Botryllus,  Pyrosoma,  colo- 
nies of  which  may  be  more  than  a  foot  long,  and  Salpa, 
the  chains  of  which  are  sometimes  several  feet  long. 

0.  In  the  true  Vertebrata  the  anterior  end  of 
the  central  nervous  system  is  enlarged  into  a  brain, 
which  becomes  surrounded  and  protected  by  a  carti- 
laginous capsule  or  skull ;  supporting  and  protect- 
ing arches,  which  finally  become  distinct  vertebrae, 

are  developed 
around  and 
above  the  noto- 
chord,  which,  in 
the  adults  of  the 

Pig.  44.— Pyrosoma ;  A,  The  atrial  or  excurrent       higher        forms, 

opening-  is  completely 

aborted.  Optic, 

auditory,  and  olfactory  organs  are  developed;  there 
is  a  centralised  heart  and  a  distinct  liver  appended  to 
the  enteric  tract.  They  are  divisible  into  two  groups, 
distinguished  by  the  fact  that,  in  the  higher,  an  an- 
terior gill-arch  becomes  modified  to  form  jaws  at  the 
sides  of  the  mouth. 

o.  Cyclostomata,  or  Round-Mouths ;  these  are 
the  lampreys  (Petromyzon),  and  hags  (Myxine).  There 
is  here  no  maiidibular  arch,  no  appendages  in  the 
form  of  limbs,  and  the  olfactory  organ  is  single  and 
median.  The  hags  are  parasitic  in  habit. 

ft.  Gnathostomata.— In  this  division  all  the 
remaining  Yertebrata  are  included ;  in  them  an  ante- 
rior gill-arch  becomes  niandibular,  two  pairs  of 
lateral  appendages  are  typically  developed,  and  the 
nasal  sac  is  double. 

In  all  divisions  of  the  animal  kingdom  we  may 
observe  groups  which  seem  to  stand  near  the  ancestral 


Chap.  III.] 


ICHTHYOPSIDA . 


forms,  and 
others  in 
which,  a  given 
complexity  of 
structure  hav- 
ing been  at- 
tained, there  is 
a  profusion  in 
the  elaboration 
of  the  details. 
This  truth  is 
well  exempli- 
fied in  the 
groups  of  the 
Vertebrata. 

I.  Ichthy- 
opsida;  these 
are  the  true 
Fishes,  and  the 
Amphibia  (or 
frogs  and 
newts).  In 
them  respira- 
tion is  always 
effected  by  gills 
during  some  or 
the  whole  of 
their  life,  the 
heart  never 
has  more  than 
three  cham- 
bers, and  there 
are  always  two 
aortic  arches  at 
least  given  off 
from  it. 

a.  Pisces. 


90    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

1.  Elasmobranchii     (sharks    and    rays). — In 
"  cartilaginous  fishes  "  the  gill  slits  are,  in  the  simples^ 
naked,  i.e.  not  covered  over  by  any  fold  (opercnluni), 
neither  the  skull  nor  the  jaws  are  ever  protected  by 
ossifications  of  the  investing  membrane  (membrane 
bones) ;    the  notochord   has   the  outer  sheath    pro- 
vided with  rings  of  ossification,  or  distinct  vertebrae 
become   developed.     The   skin    is    either    naked,    or 
covered  with  calcified  tooth-like  papillae. 

2.  Dipnoi;     e.g.     Lepidosiren,     Ceratodus. — In 
these  the  cartilaginous  brain  capsule  becomes  invested 
by  bones  developed  in  the  covering  membrane,  and 
the  digestive  tract  gives  off  a  single  or  incompletely 
divided  air  sac,  which  is  more  or  less  richly  supplied 
with  blood-vessels,  and  may  undertake  the  office  of  a 
lung,  the  possession  of  which  enables  the  fish  to  live 
in  mud.     The  pectoral  and  pelvic  tins  are  broad  and 
paddle-like  (Fig.  45),  or  elongated  and  filiform. 

3.  The  Ganoidei    and  (4)  Teleostei  are   the 
two  groups  of  the  Pisces  in  which  we  observe  that 
elaboration    of  the    details   to    which    reference   has 
already  been  made;  a  cod,  a  sole,  or  an  eel    stand 
almost  as  far  from  the  primitive  vertebrate  as  the 
snake,  the    hawk,   or  the   bat.      The  former    group 
retains  certain  more  primitive  characters  which  are 
only  rarely  or  rudimentarily  possessed  by  the  latter  ; 
thus   the   arterial   trunk    (see   page   195),    which    is 
muscular  and  contractile  in  Ela^mobranchs,   Dipnoi, 
and  Amphibians,  is  so  also  in  Ganoids,  but  is  only 
incompletely  so  in  some  Teleostei  (Butirinus) ;    the 
spiral   valve   which    is    found   in    the    intestine    of 
Elasmobranchs  is  retained  in  the  Ganoids,  though  not 
well  developed  in  the  Sturgeon  and  its  nearest  allies ; 
it  is  lost  in  most  Teleostei,  though  found  in  Butirinus 
(Stannius),  in  Chirocentrus,  and  perhaps  represented 
in  rudiment  in  the  smelt  (Huxley). 

In  both  groups  the  ends  of  the  gills  are  free,  and 


Chap.  III.] 


G A  NO  I  DEI. 


91 


the  gill  chamber  is 
covered  in  by  a 
bony  plate,  oper- 
<  n lima  ;  the  renal 
ducts  do  not  open 
into  a  depression 
(cloaca,)  common 
to  them  and  the 
anus.  In  all  Ga- 
noids, and  in  one 
great  division  of 
the  Teleostei,  the 
air  sac  on  the  dor- 
sal surface  of  the 
body  opens  by  a 
duct  into  the  oeso- 
phagus. 

The  recent  Ga- 
noidei  fall  into  two 
divisions : 

a.  Selachoi- 
dei ;  *  such  are 
the  Sturgeons  (Aci- 
penser)  and  Poly- 
odon  ;  in  these  the 
skull  consists  of 
persistent  cartilage, 
overlaid  by  bones 
developed  in  the 
investing  mem- 
brane ;  spiracles 
are  persistent,  and 
the  body  is  either 
naked,  or  has  bony 
plates  developed  in 
the  dermis  (Fig.  46). 

*  Chondrostei. 


92    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

0.  Teleostoidei ;  *  represented  by  the  North 
American  bow-fin  (Anna),  and  gar-pike  (Lepidosteus), 
and  the  North  African  Polypterus ;  in  these  the 
hinder  part  of  the  cartilaginous  cranium  always  under- 
goes ossification,  the  spiracles  close  up,  or  are  covered 
by  a  bony  plate,  and  the  scales,  which  are  never 
formed  of  true  bone,  are  large,  and  may  be  covered  by 
a  layer  of  enamel.  Vertebrae  are  developed  around 
the  notochord. 

The  Teleostei,  or  bony  fishes,  always  have  ossi- 
fied vertebral  centra,  and  more  or  less  of  the  primitive 
cranial  cartilage  is  finally  replaced  by  bone ;  scattered 
bony  plates  are  developed  in  the  dermis,  or  the  in- 
tegument is  protected  only  by  thinner  scales,  or  the 
body  is  naked  ;  they  are  divisible  into  : 

a.  Physostomi,  where  the  air  bladder,  which  is 
an  outgrowth  of  the  oesophagus,  almost  always  remains 
connected  with  it  by  an  open  duct,  and  the  hinder 
pair  of  fins,  if  retained,  as  in  the  salmon,  are  always 
abdominal  in  position ;  here  we  find  catfishes  (Silurus), 
carps,  pikes  (Esox),  and  salmons,  as  well  as  the  finless 
eels. 

£.  Pliysoklisli. — In  these  the  air  bladder  be- 
comes shut  off  from  the  oesophagus,  or  is  aborted,  as 
in  the  sole ;  the  ventral  fins,  which  are  rarely  ab- 
dominal (Notacanthus),  are  ordinarily  thoracic  or 
jugular  in  position ;  not  unfrequently  they  are  rudi- 
mentary or  lost.  The  fin-rays  are  either  all  jointed 
as  in  the  cod,  or  some  are  entire,  as  in  the  perch. 
Some  forms  are  asymmetrical  and  flattened  like  the 
sole  ;  some  swollen  and  globular  like  the  sun-fish  ;  some 
greatly  elongated  like  the  pipe-fish;  some  with  a 
prehensile  tail  like  the  sea-horse  ;  some  have  the  body 
scaleless  ;  others,  like  Diodon,  have  erectile  spines  ; 
some  can  live  in  semi-fluid  mud  (Ophiocephalus) ; 

*  Holostei. 


Chap,  in.]  AMPHIBIA.  93 

some  can  make  overland  journeys,  and  go  up  inclined 
surfaces,  if  not  trees,  like  Anabas;  some  can  take 
leaps  out  of  the  water,  like  the  "  flying  gurnards " 
(and  the  physostomous  Exocoetus) ;  some,  like  Chseto- 
don,  have  a  minute  mouth,  while  the  sword-fish  has 
its  upper-jaw  converted  into  a  powerful  piercing 
organ,  and  another  (Toxotes)  has  acquired  the  habit 
of  throwing  a  drop  of  water  at  the  insect  it  desires  to 
obtain.  Other  examples  might  be  given  of  the  pro- 
fusion of  variation  within  the  limits  of  Teleostean 
organisation. 

Even  the  lowest  of  the  Amphibia  are  dis- 
tinguished from  the  highest  of  fishes,  such  as  Cera- 
todus  or  Lepidosiren,  by  the  fact  that  their  fore  and 
hind  limbs  are  arranged  on  the  same  plan  as  in  the 
higher  vertebrata  (see  page  350),  and  these  limbs 
terminate  typically  in  five  digits,  so  that,  like  the 
higher  forms,  they  are  pentadactyle ;  if,  further, 
fins  are  developed,  they  never  have  fin-rays. 

1.  Uroclela;    in   the   lowest   of   these   (Proteus, 
Menobranchus)  (Fig.  47)  external  gills  persist  through- 
out life;  in  the  next  grade   (Amphiuma,  Menopoma) 
the  gills  are  lost,  but  the  gill-clefts  remain ;  while  in 
the  highest  (Salamandra,  Triton)  the  gills  disappear 
in  the  adults,  and  the  clefts  close  up.      All  retain  the 
tail,  which  in  the 

2.  Anura   (or   frogs    and    toads)    is   only   found 
during  the  tadpole  stage,   when   also   respiration   is 
effected  by  external  or  internal  gills,  which  disappear 
in  the  adult,  to  be  functionally  replaced  by  lungs. 

3.  Caeeilise    are  still   more    modified  forms,  in 
which  the  limbs  are  lost,  and  the  body  is  elongated 
and  serpentiform. 

The  two  higher  divisions  of  the  Vertebrata  are 
the  Sauropsida  and  the  Mammalia,  which  may 
be  grouped  together  as  the  Amniota.  They  are 
characterised  by  the  very  early  development  of  a 


94 


Chap,  in.]         SAUROPSWA  ;  MAMMALIA.  95 

large  sac-like  structure  similar  in  origin  and  primitive 
position  to  the  bladder  of  the  frog;  this  allantois 
takes  on  respiratory  functions  in  the  developing  reptile 
or  bird,  and  a  nutrient  one  in  the  higher  Mammalia. 
From  either  end  of  the  body  there  grows  out  a  fold, 
which  passes  over  the  body  of  the  embryo  and  unites 
above  it  with  its  fellow ;  this  fold,  which  is  double, 
forms  the  amnion;  the  two  layers  of  the  amnion 
separating  from  one  another  give  rise  to  a  cavity 
between  them  which  is  more  or  less  occupied  by  the 
allantois  ;  in  the  Bird  the  allantois  is  comparatively 
larger  than  it  is  in  the  Mammal. 

The  differences  between  the  Sauropsida,  or  reptiles 
and  birds,  and  the  Mammalia  are  well  and  sharply 
marked,  and  it  is  almost  impossible  to  suppose  that 
their  common  ancestor  was  not  more  amphibian  than 
amniote  in  character.  Thus,  the  Sauroids  have  scales 
or  feathers,  the  Mammals  hairs ;  the  skull  is  always 
articulated  to  the  atlas  by  a  single  condyle  in  the 
Sauroid,  and  by  two  in  the  Mammal ;  the  quadrate 
bone,  which  is  external  to  the  ear  in  the  Sauroid,  is 
enclosed  by  the  otic  capsule  in  the  Mammal  \  the  red 
blood  corpuscles  of  a  Sauroid  are,  and  of  a  Mammal 
are  not,  nucleated  ;  the  connection  between  the 
cerebral  hemispheres  of  a  Mammal  is  more  intimate 
than  in  a  Sauroid,  and  while  the  eggs  of  the  latter  are 
large,  and  provided  with  a  quantity  of  yolk,  those  of 
the  Mammal  are  much  smaller,*  and  nutrition  is 
afforded  to  the  young  by  milk,  the  secretion  of  certain 
modified  tegumentary  glands. 

The  recent  investigations  of  palaeontologists  have 

*  It  has  been  recently  stated  that  the  ova  of  the  lowest 
Mammals  are  large,  and  that  they  are  hatched  outside  of  the  body. 
This  observation,  coupled  with  the  facts  that  certain  fossil  Reptiles 
(TheriomOrpha)  give  well-marked  indications  of  mammalian  affini- 
ties, and  that  some  Reptiles  (e.g.  some  of  the  Amphisbsenidae) 
have  the  occipital  condyle  double,  may  necessitate  a  revision  of 
current  ideas  as  to  the  origin  of  the  Mammalia. 


96    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

afforded  us  a  complete  series  of  intermediate  stages 
between  the  reptiles  and  birds,  and  they  are  justly 
united  in  the  common  group  of  the  Sauropsida. 

A.  Reptilia.  —  Sauroids  with  horny  or  bony 
plates,  but  no  feathers,  with  more  than  three  digits  in 
the  manus,  of  which  three  at  least  bear  claws,  with  at 
least  three  digits  in  the  pes,  and  with  unankylosed 
metatarsals.  The  blood  is  ordinarily  cold,  and  there 
is  at  least  one  pair  of  aortic  arches. 

1,  2.  Lacertilia,  or  lizards,  and  Op  hi  ilia,  or 
snakes,  have  the  quadrate  movable,  the  penis  double, 
and  the  anus  a  transverse  slit.  Some  of  the  Lacer- 
tilia, such  as  Lacerta  (the  common  lizard),  are 
the  least  modified  of  all  Sauroids,  and  the  Geckos 
retain  a  primitive  character  in  the  persistence  of 
remnants  of  the  notochord.  Others  are  specially 
modified,  like  the  flying  lizard  (Draco),  others  have 
ossified  scutes  approaching  those  of  crocodiles  (e.g. 
Cycled  us) ;  Hatteria  is  remarkable  for  the  possession 
of  "  uncinate  processes  "  on  the  ribs  (see  page  346), 
such  as  are  seen  in  crocodiles  and  birds.  Some,  like 
the  blind-worm,  lose  their  limbs,  but  all  have  a  pectoral 
arch  and  a  urinary  bladder,  both  of  which  are  absent 
from  the  Opliiclia,  in  which  the  hind  limbs  are  rarely 
present,  and  then  are  only  short  and  inconspicuous. 
They  are  divisible  into  the  Eurystomata,  in  which 
the  mouth-cavity  is  capable  of  dilatation,  and  the 
Stenostomata,  in  which  the  facial  bones  are  im- 
movably connected  with  one  another.  Among  the 
former  we  find  vipers,  rattlesnakes,  and  water  snakes, 
which  are  venomous  ;  and  adders,  boas,  and  pythons 
which  are  not  so.  Typhlops  and  Uropeltis  are 
examples  of  the  Stenostomata. 

3.  Chelonia,  or  turtles  and  tortoises. — In  these 
the  quadrate  is  immovably  connected  with  the  side 
of  the  skull,  the  penis  is  simple  and  solid,  and  the 
anal  orifice  rounded.  The  bony  plates  developed  in 


Chap,  in.]  REPTILES  :  BIRDS.  97 

the  dermis  are  definitely  arranged,  and  form  a 
"  carapace,"  which  is  generally,  though  not  always 
(Trionyx),  covered  by  horny  epidermic  plates,  which 
form  the  "  tortoise-shell."  They  exhibit  a  primitive 
character  in  the  retention  of  the  five  digits  in  either 
limb,  but  diverge  from  the  typical  organisation  in  the 
loss  of  teeth  ;  an  interesting  series  of  modifications,  in 
relation  to  their  mode  of  life,  are  exhibited  by  the 
limbs.  .In  the  tortoises,  which  are  terrestrial,  the 
digits  are  free  ;  in  the  amphibian  terrapenes  there  is  a 
partial  web,  which  is  more  complete  in  the  Triony- 
chidse  ;  while  the  marine  Cheloniidse  have  the  digits 
completely  covered  by  skin,  so  that  they  form 
flattened  swimming  fins. 

4.  Crocodilia,  or  crocodiles  and  alligators,  are 
the  only  reptiles  in  which  the  heart  is  four-chambered  ; 
like  the  Chelonia,  they  have  the  quadrate  immovably 
connected  with  the  side  of  the  skull,  the  penis  is 
simple  and  solid,  and  the  anal  orifice  is  rounded.  The 
teeth  are  set  in  distinct  sockets,  and  are  never  found 
on  any  bones  but  the  maxillae,  premaxillae,  and 
dentaries.  They  have  returned  to  an  amphibious  or 
aquatic  mode  of  life,  in  correlation  with  which  their 
feet  are  webbed,  the  nostrils  can  be  closed,  and  the 
tympanic  membrane  of  the  ear  covered  over. 

B.  Aves,  or  birds,  are  Sauroids  with  feathers, 
with  never  more  than  three  digits  in  the  manus,  or 
four  in  the  pes  ;  three  of  the  metatarsals  are  ankylosed 
with  one  another,  and  with  the  distal  tarsal  bone. 
The  blood  is  hot,  and  there  is  only  a  single  systemic 
aorta.  All  recent  forms  are  toothless.  Physiologically, 
if  not  also  morphologically,  the  recent  forms  are 
divisible  into  : 

I.  Ratitae,  in  which  the  ventral   surface  of  the 
sternum  is  broad  and  flattened,  and  the  fore-limb  does 
not  form  a  functional  wing ;  such  are  the  ostrich  and 
the  cassowary. 
H— 16 


98    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

II.  Cariiiatse,  in  which  the  ventral  surface  of 
the  sternum  is  typically  provided  with  a  median  keel, 
and  the  fore-limbs  may  serve  as  functional  wings. 
Among  them  we  find  the  singing  birds,  parrots,  owls, 
eagles,  geese,  pigeons,  and  gulls  ;  and  here,  as  among 
the  Teleostei,  we  find  the  most  varied  elaborations  in 
the  details  of  a  structural  organisation,  which  is,  in  its 
essential  points,  extraordinarily  similar  throughout 
the  group.  The  extinct  Odontornithes  (e.g.  Hes- 
perornis)  were  true  birds  with  teeth  in  their  jaws. 


The  Ulammalia,  or  last  division  of  the  Verte- 
brata,  are  all  distinguished  from  the  Sauropsida  by 
the  possession  of  two  occipital  condyles,  and  by  the 
fact  that  the  single  aortic  arch  is  the  left  and  not  the 
right  member  of  a  primitive  pair.  They  are  all  more 
or  less  hairy,  and  have  mammary  glands ;  the  quad- 
rate becomes  the  malleus  among  the  auditory 
ossicles,  the  blood  is  hot>  and  the  red  blood  corpuscles 
are  without  a  nucleus,  while 
the  cerebral  hemispheres  have 
a  corpus  callosum.  (See 
page  426.)  They  exhibit  three 
Well-marked  grades  of  develop- 
ment : 

A.  Prototheria  (Ornitlw- 
delphia),  in  which  the  mammary 
glands  are  without  teats,  the 
young  are  not  nourished  within 
the  uterus  of  the  mother  by 
means  of  a  placenta*  the 
epipufoes  (see  page  348)  are 

larSe'  and  the  coracoids 
complete.  Here  are  placed  the 
duck-bill  (Ornithorhynchus)  and  the  Echidna  (Fig.  48), 
which  have  so  far  diverged,  like  the  Chelonia,  from 


Chap,  in.]  MAMMALIA.   '  99 

the    primitive    type,    that    they    are    without    true 
teeth. 

B.  Metatheria   (Didelphia).  —  These     are     the 
Marsupials;  they  have  true  teats,  but  no  placenta ; 
the  epipufoes  are  large,  but  the  coracoicl  rudimen- 
tary.    The  Marsupials  exhibit  a  great  range  of  varia- 
tion and  structure  among  themselves ;  some  are  car- 
nivorous,  like  the   Opossum,   the  Dasyurus,  and  the 
Thylacine ;    others   herbivorous,    like    the     kangaroo 
(Macropus)  and  the  wombat  (Phascolomys). 

C.  Eutheria  (Monodelphia).  —  Here  stand  the 
rest  of  the  Mammalia,    which,    without  any  known 
exception,  have  teats,  a  placenta,  rudimentary  or 
no  epiptifoes,  and  a  rudimentary  coracoid.     The 
least  differentiated  are  the  Insectivora  (e.g.  hedgehog, 
mole),  to  which  are  most  closely  allied  the  Chiroptera 
(bats),  and  the  Rodents   (rat,   rabbit) ;  in  these  the 
yolk  sac  takes  a  larger  share  in  the  formation  of  the 
placenta  than  it  does  in  other  mammals. 

The  Edentata  form,  at  the  present  day,  an  isolated 
group,  represented  by  the  sloths,  anteaters,  and  arma- 
dillos, by  the  pangolins  (Manis),  and  by  the  ant-bear 
(Orycteropus).  The  hoofed  animals,  or  Ungfulata, 
form  a  well-marked  division,  in  which  the  group  of 
the  even-toed  forms  (Aitiodactyla),  such  as  the 
pig,  deer,  and  cow,  is  very  distinct  from  that  of  the 
odd-toed  (Perissodactyla),  such  as  the  tapir,  rhino- 
ceros, and  horse.  With  the  Ungulata,  the  coney 
(Hyrax)  and  the  elephant  may  be  associated  (Flower). 
Of  aquatic  forms,  the  Cetacea,  or  porpoises,  toothed 
whales,  and  whalebone  whales  seem  to  stand  nearest 
to  the  Ungulates.  Of  the  affinities  of  the  other 
aquatic  mammals,  the  Sirenia,  or  manatee  and 
dugong,  we  can  only  with  confidence  say  that  they 
are  not  with  the  Cetacea.  The  true  Carnivora  are 
the  dogs,  cats,  and  bears,  and  with  these  are  closely 
allied  the  walruses  and  seals. 


ioo    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

By  the  almost  universal  consent  of  zoologists,  the 
highest  "  order "  of  the  Mammalia  is  that  of  the 
Primates;  of  these,  the  lowest  suborder  is  that  of 
the  L.emuroidea  (of  which  some  naturalists  would 
make  a  separate  order),  the  highest  that  of  the 
Anthropoidea,  which  is  divisible  into  five  "  fami- 
lies," the  highest  of  which  is  the  Ilomiiiidsr, 
represented  by  the  single  genus  Homo. 

While  Man  is  said  to  be  the  highest  of  animals, 
it  is  not  to  be  forgotten  that  in  the  other  divisions 
of  zoologists  there  are  forms  in  which  structural 
characters  are  at  least  as  perfectly  elaborated,  when 
we  bear  in  mind  their  ancestral  history  and  the 
relation  of  structure  to  function.  The  horse,  the 
whalebone  whale,  the  woodpecker,  or  the  boa  con- 
strictor, are,  to  cite  only  a  few  examples,  forms  in 
which  structural  organisation  is  as,  if  not  more,  com- 
plete, and  as  differentiated  as  it  is  in  man. 


There  remain  to  be  considered  very  briefly  several 
groups  of  animals  which,  in  the  present  state  of  our 

knowledge,  cannot  be 
satisfactorily  placed 
with  any  of  the  great 
phyla  which  we  have 
just  been  describing. 
Of  these  the  more  im- 
portant are  : 

1.  Bracliiopoda, 
— These  were  placed 
by  earlier  naturalists 
with  the  Mollusca,  from 
which,  however,  they 
are  to  be  distinguished 
in  consequence  of  the  segmentation  of  the  larva,  the 
dorsal  and  ventral  positions  occupied  by  the  two 


Fig.  49. — Crania  anomala.      b,  Arms. 
(After  Davidson.) 


Chap,  in.]         BRACHIOPODA:  BRYOZOA.  101 

unequal  valves  which  make  up  their  shell,  and  by  the 
characters  of  their  nervous  system.  The  so-called 
arms  (Fig.  49  ;  b)  are  outgrowths  of  the  pree-oral  disc 
of  the  larva,  at  the  edges  of  which  the  tentacles  or 
cirri  are  set.  -  This  great  development  of  their  arms  is 
to  be  correlated  with  the  fixed  habit  of  the  adult. 


Fig.  50. — Bugula  purpurotincta.     Nat.  size.     (After  Hiucks.) 

Terebratula  and   Lingula   (which  is  stalked)  are  ex- 
amples of  this  isolated  and  geologically  ancient  group. 

2.  The  Bryozoa  have  likewise  been  placed  with 
the    Mollusca;    they    are   clearly   degenerate    forms 
which,   by  the  characters  of  their  larvae,   appear  to 
have  been  descended  from    an   ancestor  common  to 
them  and   the  Chsetopoda.       Balfour    has    suggested 
that  they  become  fixed  by  their  prse-oral  lobe.     They 
live  in  colonies,  and  are  the  forms  that  are  popularly 
known  as  sea-mats  or  sea-mosses  (Fig.  50). 

3.  The  Chsetogrnatha  (as  represented  by  Sagitta) 
are  forms  that  have  relations  to  the  Chsetopoda  and 


102     COMPARATIVE   ANATOMY  AND   PHYSIOLOGY. 

to  the  round  worms,  but  differ  from  them  remarkably 
in  the  mode  of  development  of  their  body  cavity, 
which  is  an  enterocoele. 

4.  Ulyzostomiim  is  a  form  with  some  points  of 
resemblance  to  the  Chsetopoda;  its  characters,  how- 
ever, are  still  obscure,  partly,  no  doubt,  on  account  of 
its  having  taken  to  the  habit  of  living  parasitically 
on  Crinoids,  on  which  alone  it  has  as  yet  been 
detected. 


CHAPTER  IV. 

ORGANS      OF      DIGESTION. 

THE  activity  of  a  living  organism  has  for  one  of  its 
chief  results  destruction  and  loss  of  tissue ;  this  loss 
can  only  be  made  up  for  by  the  act  of  taking  in  fresh 
material  from  the  outer  world.  In  the  necessary 
nutrition  of  an  organism,  we  find  that  the  first 
process  is  that  of  digestion,  by  means  of  which 
substances  foreign  to  the  organism  become  assimi- 
lated to  it,  and  are  rendered  capable  of  being 
absorbed,  and  of  passing  into  that  stream  whence 
the  different  parts  of  a  body  take,  as  they  require, 
the  food  which  is  needed  to  make  up  the  losses  caused 
by  their  several  activities.  Organisms  are,  in  other 
words,  metabolic. 

It  is  to  be  carefully  borne  in  mind  that  the  essen- 
tial step  in  the  nutrition  of  an  animal  is  that  of 
assimilation,  and  it,  indeed,  is  the  only  process 
which  obtains  in  the  case  of  the  lowest  and  simplest 
organisms.  In  other  words,  a  simple  mass  of  proto- 
plasm, such  as  an  Amoeba,  takes  up  from  without 
food  material  into  its  own  substance,  and  this,  as 
we  have  already  learnt,  is  effected  directly ;  the 
material  thus  taken  in  is  acted  upon  by  the  living 


chap,  iv.i        INTRACELLULAR  DIGESTION.  103 

protoplasm  of  the  cell,  which  is  capable  of  separating 
out  from  the  food  such  parts  as  are  nutritious,  and 
of  converting  them  into  protoplasmic  matter ;  what 
is  useless  is  discharged,  or  got  rid  of. 

This  direct  mode  of  assimilation  by  a  living  cell  is 
spoken  of  as  intracellular  digestion;  it  is  the 
only  mode  of  nutrition  which  is  known  to  obtain 
in  the  Protozoa,  but  it  is  very  important. to  observe 
that  the  phenomenon  is  by  no  means  limited  to  that 
division  of  the  animal  kingdom  ;  it  obtains  also  in 
various  lower  groups  of  the  Metazoa,  and  even  after  a 
distinctly  defined  mouth  has  become  developed.  It 
is,  therefore,  associated  with  a  number  of  characters 
which  indicate  an  advance  in  the  complexity  of 
organisation;  and,  on  the  other  hand,  it  is  found 
also  in  forms  which  have,  under  the  influence  of  a 
parasitic  habit,  become  degraded  as  compared  with 
their  ancestors.  The  simplest  mode  of  seizing  food  is 
observed  in  the  Amoeba,  where  the  protoplasmic  body 
seems  to  engulf  its  nutriment  by  flowing  and  closing 
around  it.  And  this  ingestion  of  food  does  not 
take  place  at  any  definite  point  in  the  body  of  the 
Amoeba,  but  now  at  one  spot,  and  now  at  another. 
When  the  form  of  the  body  becomes  more  definite, 
the  protoplasmic  processes  act  as  organs  by  which 
the  food  is  drawn  towards  the  central  body-mass. 

A  much  more  elaborated  mode  is  to  be  seen  in 
the  ciliated  Infusorians,  where  a  definite  orifice 
("  cytostome ")  acts  as  the  sole  entrance  for  food 
into  the  body ;  in  many  cases  this  so-called  <l  mouth  " 
has  also  an  anal  function,  but  in  a  few  forms  it  has 
been  distinctly  observed  that  a  second  orifice  is  pre- 
sent ;  by  means  of  this  "  cytoproct,"  the  undigested 
portion  of  the  food  passes  from  the  body.  The 
presence  of  a  definite  oral  orifice  is  no  doubt  to  be 
associated  with  the  greater  elaboration  of  the  organi- 
sation of  an  Infusorian,  and  we  find  also  that  some 


io4    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

of  its  most  characteristic  organs  (the  cilia)  are 
specially  modified  in  the  neighbourhood  of  the 
"  mouth."  In  other  words,  we  have  here  the  first 
sign  of  a  correlation  between  the  digestive  orifice  and 
the  organs  which  are  locally  connected  with  it,  and 
which  are  also  in  relation  with  the  outer  world.  The 
cilia  around  the  mouth  of  Paramcecium  (Fig.  3  I.)  are 
much  longer  than  those  which  fringe  the  greater  part 
of  their  body,  and  give  rise  to  more  powerful  currents, 
by  means  of  which  food  particles  are  floated  towards 
the  orifice. 

0  Some  of  the  ciliated  Infusorians,  such  as  Opalina 
ranarum  and  Anoplophrya  are  endoparasitic,  and  in 
these  the  mouth  is  lost  (Fig.  51),  as  it  is  also  in  the 
Gregarinida,  which  live  in  cavities  rich  in  nutrient 
matter,  such  as  the  intestine  of  the  lobster,  or  the 

testicular  re- 
servoirs of  the 
earthworm ;  in 

Pig.    51.— Anoploplirya  prolifera.     (After  Clapa-     SUC^    ^Orms    ^ 
rede  and  Lachmann.)  these     nutri- 

ment    enters 

into  the  substance  of  the  cell  by  the  mere  physical 
process  of  diffusion  or  osmosis. 

In  the  ectoparasitic  Suctoria,  where  the  mouth  is 
likewise  lost  (Fig.  3  in.),  processes  of  the  body  are 
drawn  out  into  sucking  tubes  with  knobbed'ends ;  these 
tubes  retain  the  extensile  and  contractile  power  of 
simple  protoplasm,  so  that  they  are  able  to  elongate 
themselves  in  such  a  way  as  to  touch  their  prey, 
which  is  ordinarily  a  ciliated  infusorian,  and  to  con- 
tract themselves  so  as  to  draw  the  prey  nearer.  The 
knob  is  enabled  to  bore  its  way  beneath  the  cuticle, 
and  then,  in  the  words  of  Stein,  "  a  very  rapid  stream, 
indicated  by  the  fatty  particles  which  it  carries,  sets 
along  the  axis  of  the  tentacle,  and,  at  its  base,  pours 
into  the  neighbouring  part  of  the  body  of  the  Acineta." 


Chap,  iv.]        INTRACELLULAR  DIGESTION. 


I05 


No  movement  of  the  wall  of  the  tentacle  has  been 
observed,  and  the  cause  of  the  production  of  this 
stream  is  still  unknown. 

As  has  been  already  observed,  the  simplest  mode  of 
digestion  (the  intracellular)  is  not  confined  to  the 
Protozoa ;  it  has 
been  observed  in 
Sponges,  Coelen- 
terata,  and  the 
lowlier  worms. 
A  clear  idea  of 
what  is  under- 
stood by  this 
method  will  be 
obtained  from 
the  consideration 
of  a  single  case. 
When  a  section 
is  made  through 
the  body  walls  of 
a  Hydra  we 
find  that  the  en- 
dodermal  cells 
vary  cons  i  d  e  r  - 
ably  in  size,  and 
that,  while  some 
are  provided 

with  a  Single  long  Fig.  52.-Longitudmal  Section  of  the  Body  of 
,,         ,,  a  Hydra,  killed  m  full  digestion, 
tiageilum,  Others  ec<  Ectoderm;  en,  endoderm;     mp,   muscular   pro- 
are      distinctlv  cesses;    d,  a  diatom ;/,  food  particles.    (After 

amceboid  in 

form,  and  give  off  large  pseudopodia  (Fig.  52) ;  within 
these  cells  dark-coloured  granules  of  various  sizes  are  to 
be  detected,  and  these  food-particles  are  sometimes 
found  to  be  "  half  in  and  half  out  of  the  protoplasm  " 
(T.  J.  Parker).  In  such  a  form,  therefore,  as  the  Hydra, 
there  would  not  seem  to  be,  as  in  Man  and  most  of 


io6    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


the  Metazoa,  any  secretion  poured  out  into  the  gastric 
cavity  from  the  cells  which  line  this  space,  but  a 
number  of  the  cells  would  appear  to  retain  the  power 
of  separately  assimilating  the  food  material.  Obser- 
vation of  these  endoderinal  gastric  cells  shows  that  they 
vary  considerably  in  size  according  to  the  fasting  or 
well-fed  condition  of  the  animal  ;  and  we  are  entitled 
to  suppose  that  these  cells  become  smaller  after  the 
process  of  digestion,  in  consequence  of  their  having 
given  up  part  of  their  acquired  material  to  the  other 
cells  of  the  body  ;  cells  which,  be  it  understood,  have 

lost  the  power 
of  independently 
assimilating  nu- 
triment. We 
have  here  to  do 
with  nutrient 
cells,  just  as  in 
the  Ccelenterata 
(page  39)  we  ob- 
served nut  rien  t 
persons  (trophos- 
omes)ina  colony. 
A  history, 
not  unlike  that 
of  Hydra,  may 
be  told  of  a 
Sponge;  but 
here,  it  is  interes- 

Fig.  53.—  Flagellated  Chambers   (c)   of  Turkey     ,«         .„         , 

tmg  to  note,   we 

have  to    do   not 

with  an  amceboid 

ingestive  cell, 
but  with  another  form  which,  no  less,  has  its  representa- 
tive among  the  Protozoa  ;  we  find,  that  is,  a  "  collared- 
cell"  taking  the  place  of  the  amoeboid  cell  (Fig.  53).  In 
the  "ciliated  "or  "flagellated  "chambers  which  are  found 


Bath  Sponge,  showin?  the  collared-cells  and 

flageiium. 

K,  Excurrent  canal  s  ^  Current  canals.  (After 


Chap,  iv.]         INTRACELLULAR  DIGESTION.  107 

along  the  course  of  the  canals  which  traverse  the  body 
of  a  sponge  we  find  a  single  layer  of 'cells,  each  of  which 
is  provided  with  a  long  whip-like  process  (flagellum), 
and  has  the  free  edge  of  its  protoplasm  converted  into 
a  collar-like  fringe.  By  the  action  of  the  flagellum 
currents  are  set  up  around  the  cell,  and  directed  to  the 
space  surrounded  by  the  collar ;  these  currents  of 
water  bear  with  them  minute  food-particles,  which 
thus  make  their  way  into  the  substance  of  the  cell. 
Such  flagellate  cells  recall  the  Flagellate  Infusoria, 
among  the  Protozoa.  Finally,  in  the  case  of  the  lower 
worms,  we  have  the  evidence  which  is  afforded  by 
Mesostomum  ehrenbergii,  a  Turbellarian  which  lives 
on  the  small  Annelid  Nais.  In  observations  on  intra- 
cellular  digestion  no  method  is  more  fruitful  in  its 
results  than  that  which  consists  in  feeding  an  animal 
with  some  finely-divided  colouring  matter  such  as 
carmine ;  Mesostomum,  however,  has  been  found  to 
reject  this  substance,  and  the  ingenious  expedient  had 
to  be  resorted  to  of  first  feeding  the  Nais  with  carmine, 
and  then  inducing  the  turbellarian  to  eat  the  annelid. 
This  experiment,  which  was  completely  successful, 
afforded  certain  evidence  as  to  the  persistence  of  the 
intracellular  mode  of  digestion  in  this  animal,  for  a 
large  quantity  of  the  coloured  material  was  found  in  its 
digestive  cells. 

The  phenomenon  of  intracellular  digestion  has 
been  now  seen  to  be  very  widely  distributed  among  the 
lower  Metazoa,  and  observations  are  continually  being 
made  in  confirmation  of  the  facts  here  described. 
With  a  single  exception,  no  observer  has  as  yet  seen 
any  combination  of  this  primitive  method  of  taking  in 
food  with  the  more  complex  one  of  the  presence  of  a 
set  of  cells  which  secrete  a  special  gastric  juice ;  we 
may  expect,  however,  to  find  that  the  sharp  distinction 
between  the  lower  and  the  higher  methods  will  be 
bridged  over  by  other  forms  than  the  fresh-water 


io8    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Medusa  (Limnocodium) ;  in  this  remarkable  creature 
the  cells  which  line  the  mouth  of  the  gastric  tube  have 
the  function  of  secreting  cells,  while  it  is  only  in  those 
that  lie  at  the  opposite  end  of  the  tube  that  the  intra- 
cellular  method  has  been  observed  (Lankester). 

This  power  of  intracellular  digestion  is  not  con- 
fined to  the  cells  that  line  the  gastric  or  endodermal 
cells  ;  Metschnikoff  has  observed  that  some  of  the 
organs  (nematocalyces)  of  the  hydroid  Plumularia 
may  be  fed  with  powdered  carmine,  when  the  dust  will 
enter  into  the  substance  of  the  cells  of  the  ectoderm, 
which,  like  the  endodermal  cells  of  Hydra,  have  re- 
tained the  power  of  protruding  pseudopodia.  The 
mesoderm,  likewise,  in  the  form  of  the  wandering  cells 
of  sponges,  and  in  the  larvae  of  Echinoderms,  where 
some  of  the  organs  disappear,  or  are  not  continued  on 
into  the  structure  of  the  adult,  exhibits  this  same 
property.  Even  in  the  higher  Metazoa  the  white 
blood  corpuscles  have  been  observed  by  Koch  to  have 
in  their  midst  bacilli  which  they  have  taken  into 
their  own  substance  ;  and  in  inflammatory  processes 
large  connective  tissue-cells  may  be  observed  eating  up 
blood  corpuscles,  carmine  granules,  and  pigment 
particles. 

The  lowest  and  simplest  condition  of  the  wall  of 
the  gastric  cavity  is  to  be  seen  in  the  lowest  Ccelenterata, 
which  present  a  far  more  primitive  arrangement  than 
do  most  of  the  sponges  ;  there  is,  indeed,  hardly  a 
perceptible  advance  on  what  is  found  in  the  typical 
gastrula,  and  such  as  there  is,  is  due  to  the  presence  of 
the  tentacles  around  the  mouth ;  the  central,  or  axial 
sac,  lined  with  endodermal  cells,  is  continued  into  the 
tentacles. 

If  we  bear  this  arrangement  carefully  in  mind,  we 
shall  be  able  to  refer  to  it  the  greater  number  of 
arrangements  which  are  to  be  found  in  the  higher 
Ccelenterates ;  we  have,  in  other  words,  to  look  for  an 


Chap.  IV.]  CCELENTERATA.  1 09 

axial  gastric  cavity,  with  which  there  communicate 
passages  or  canals.  The  stomach  may  be  enlarged  in 
some,  and  diminished  in  other  directions,  and  the 
canals  may  be  greatly  developed  in  number,  and  pro- 
vided  with  outgrowths  or  pouches ;  but  the  essence  of 
the  arrangement  is  still  apparent. 

When,  as  so  frequently  happens,  a  number  of 
hydroid  polyps  become  connected  with  one  another  by 
a  common  trunk  and  form  a  colony,  the  gastric  cavity 
of  each  polyp  is  brought  more  or  less  into  relation 
with  those  of  the  rest ;  for  each  cavity  is  continuous 
with  the  canal  which  runs  in  the  centre  of  the  stem 
or  trunk  of  the  colony  and  the  cells  which  line  this 
passage  are  provided  with  cilia.  The  facts  that  some 
polyps  occupy  positions  moie  easily  accessible'  to  food 
currents  than  others,  and  that  the  less  fortunately 
situated  can  draw  on  theii  fellows,  lead,  in  a  number  of 
cases,  to  a  division  of  labour  ,  those  best  adapted  for  the 
business  of  nutrition  come  to  limit  their  activities  to 
this  important  duty  (trophosomes),  while  others, 
fed  at  their  expense,  devote  themselves  to  the  equally 
important  duty  of  developing  the  generative  products, 
and  so  take  on  the  especial  function  of  reproducing  the 
species  (gonosomes).  The  Stylasteridae,  on  the  other 
hand,  afford  us  examples  of  zooids  which,  having  ceased 
to  be  nutrient,  have  become  reduced  to  mere  tentacles, 
the  duties  of  which  they  alone  perform  (dactylo- 
zooids). 

It  is  now  necessary  to  direct  attention  to  a  portion 
of  the  gastric  apparatus  of  Hydra,  which  was,  for  the 
moment,  neglected  ;  the  mouth  of  Hydra,  or  indeed  of 
any  hydroid,  is  not  a  mere  space  in  the  wall  of  the 
body,  but  forms  a  conical  process,  at  the  tip  of  which  is 
set  the  orifice,  so  placed  that  when  a  hydra  is  looked 
at  from  the  side,  the  mouth  cone  only  can  be  seen,  and 
the  wide  mouth  itself  is  hidden. 

If  we  pass  now  to  the  other  extreme  of  the  series 


no    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

and  examine  a  free-swimming  Medusa  (Fig.  10)  we 
find  that  the  mouth  and  the  stomach  form  a  free 
projection  hanging  downwards,  sometimes  in  the  shape 
of  a  tube  of  some  length  \  around  this  mouth  we  again 
find  tentacles,  and  if  we  examine  the  first  portion  of 
the  gastric  apparatus  we  find  it  is  widened  out  to  form 
what  may  be  called  a  stomach  ;  connected  with  this 
last  there  are  a  large  number  of  canals  which  channel 
the  substance  of  the  disc  of  the  umbrella,  and  carry 
into  it  the  nutriment  prepared  by  the  gastric  cells ; 
these  canals  have,  therefore,  a  circulatory  function,  and 
are,  consequently,  appropriately  spoken  of  as  the 
gastrovascular  canals.  They  either  run  simply 
or  are  ramified,  and  are  again  brought  into  connection 
with  one  another  by  opening  into  a  canal  which  runs 
round  the  edge  of  the  umbrella ;  from  this  canal 
cavities  sometimes  pass  into  the  tentacles  which  fringe 
the  margin  of  the  disc. 

The  tentacular  processes  set  around  the  mouth  are 
often  of  considerable  size,  and  are  in  certain  forms 
broken  up  into  a  number  of  processes ;  in  one  group 
(that  of  the  Rhizostomidse)  this  is  carried  to  an  ex- 
treme, for  the  oral  tentacles  take  the  place  of  the 
mouth,  which,  in  the  adult,  is  closed  up,  and  they 
become  provided  with  digestive  cells  and  openings  to 
the  exterior  ;  so  that  in  these  forms  a  number  of  small 
secondary  orifices  take  the  place  of  the  single  large 
primitive  mouth. 

The  other  great  division  of  the  Ccelenterata,  that 
of  the  Anthozoa,  presents  us  at  once  with  an  impor- 
tant distinctive  character ;  for  the  mouth  is  not  placed 
on  a  projecting  cone,  but  is  depressed  below  the  level  of 
the  surrounding  platform  developed  from  the  body  wall 
(Fig.  54).  The  second  distinction  is  perhaps  the  more 
important ;  the  tube  into  which  the  mouth  leads  is 
widely  open  at  the  lower  end  ;  in  other  words,  we 
have  here  the  appearance  not  of  a  system  of  canals 


Chap.  IV.] 


ANTHOZOA. 


in 


JLbt 


channelling  the  surrounding  tissue,  but  rather  of  a 
series  of  chambers  separated  from  one  another  by  nar- 
row septa,  while  even  these  are  perforated  by  two 
holes  (Fig.  54 ;  I}. 

The  mouth  is  an  elongated  slit,  which  sometimes 
becomes  constricted  in  its  middle,  so  that  we  have 
essentially  two  orifices. 
On  the  ventral  side  of  this 
slit  a  groove  is  often  deve- 
loped, which  leads  into  the 
gastric  cavity ;  the  cells 
which  line  the  sides  of 
this  groove  (the  "  siphono- 
glyphe"  of  Hickson)  (Fig. 
55  ;  st),  are  ciliated,  and  by 
the  action  of  these  cilia 
the  food  is  carried  to  the 
digestive  region  of  the 
body  ;  the  presence  of  this 
groove  or  the  size  to  which 
it  is  developed  have  been 
observed  to  vary  with  the 
size  of  the  animal,  or  of 
the  colony  of  which  the 
polyp  is  a  part;  or,  in 
other  words,  to  depend  upon  the  demand  for  food 
which  is  made  by  the  Alcyonarian  (Fig.  55). 

The  great  size  of  this  mouth  slit,  and  the  fact  that 
it  is  often  constricted  in  its  middle,  are  of  considerable 
Importance  as  bearing  on  the  early  history  and  func- 
tion of  the  blastopore,  or  opening  into  the  gastrula  ; 
in  simple  or  archaic  forms,  such  as  Peripatus,  the 
blastopore  is  a  greatly  elongated  slit  which  closes  up 
in  the  middle,  and  forms  the  mouth  at  one  end  and 
the  anus  at  the  other. 

ft        In    the   Anthozoon    Peachia    the   mouth    slit   is 
similarly  converted  into  two  openings,  one  of  which 


artia 


Tentacle ;  I,  internal  septal  stoma ; 
Im,  longitudinal  muscle  ;  tm,  trans- 
verse muscle  ;  pm,  parietal  mus- 
cle ;  v,  mesenterial  filaments ;  w, 
Acontia.  (After  O.  and  R.  Hertwig.) 


1 12    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

has  the  function  of  an  ingestive,  and  the  other  of  an 
egestive  passage  (Sedgwick). 

The  walls  of  the  digestive  tract  are  not,  as  in  the 
Hydrozoa,in  direct  contact  with  those  of  the  body  ;  the 


Fig.  55. — Transverse  Section  of  a  Polyp  of  Coelogorgia  plumosa, 
showing  the  long  delicate  cilia  of  the  siphonoglyphe  ($t).  (After 
Hickson.) 

intermediate  space  is  traversed  by  delicate  plate-like 
septa,  some  of  which  extend  across  the  whole,  and  others 
only  partly  project  into  the  perigastric  cavity  (Fig. 
54).  The  axial  gastric  space  communicates  at  its 
lower  end  with  the  compartments  of  the  perigastric  ; 
and  the  septa  project  more  or  less  inwards  at  this 
point.  Along  the  free  edges  of  these  septa  there  are 
placed  special  filamentous  structures,  which  are  known 
as  the  HEeseiiterial  filaments,  the  name  of  mesen- 
tery being  applied  to  such  septa  as  reach  the  walls  of 
the  gastric  tube.  The  only  physiological  experiments 
yet  made  on  those  filaments  are  those  of  Krukenberg, 
which  demonstrate  that  these  constituent  cells  act  on 


Chap,  iv.]  DIGESTION  IN  METAZOA.  113 

proteids  by  the  method  of  in  trace!  lular  digestion, 
and  they  appear  to  be  the  only  part  of  the  organism 
which  is  entrusted  with  this  duty. 

The  Ctenophora  have  the  spaces  in  connection 
with  the  axial  gastric  cavity  narrowed  to  four  canals, 
and  there  are  two  pores  at  the  aboral  pole  of  the  body. 
There  are  never  more  than  two  long  tentacles,  and 
when  these  are  lost,  as  in  Beroe,  the  mouth  is  much 
wider  than  in  the  tentaculate  forms. 

For  the  rest  of  the  Metazoa,  with  the  exception  of 
the  already  mentioned  Turbellaria  and  Trematoda 
(e.g.  liver  fluke),  the  intracellular  mode  of  digestion 
has  not  been  observed.  As  in  some  of  the  Coelenterata, 
we  have  a  higher  mode ;  the  cells  of  the  endodermal 
lining  of  the  gastric  tube  have  now  ceased  to  act  in- 
dependently of  one  another;  certain  of  them  are 
set  apart  for  the  function  of  secreting  a  ferment, 
which,  passing  from  them  into  the  digestive  cavity, 
there  acts  upon  the  food  ;  the  albuminoids  contained 
in  it  are  converted  into  substances  capable  of  passing 
through  the  wall  of  the  intestine.  Special  salivary 
glands  are,  in  many,  developed  for  the  purpose  of  con- 
verting starch  into  sugar.  There  is  some  evidence, 
however,  that  certain  cells  continue  to  take  up  nutri- 
ment into  their  own  substance ;  even  in  the  frog  some 
of  the  cells  of  the  small  intestine  have  been  observed 
to  send  out  short  processes  into  the  enteric  cavity 
(Thanhoffer),  recalling  thereby  the  amoeboid  cells  and 
the  intracellular  mode  of  digestion  which  is  seen  in 
Hydra. 

Among  the  flat-worms  we  need  here  only  consider 
the  Turbellaria  and  flukes,  as  the  tapeworms  obtain 
their  nutriment  in  a  very  special  way.  (See  page  177 ; 
Digestion  of  parasitic  animals.)  A  mouth  is  always 
present,  but  is  by  no  means  constant  in  position,  as 
it  may  be  far  forwards,  at  the  middle  of  the  body, 
or  far  back  (Opisthomum).  In  a  number  there  is 
1—16 


ii4    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

a  protrusible  proboscis,  formed  from  the  anterior 
portion  of  the  digestive  tract;  in  some  the  mouth 
does  not  lead  into  any  distinct  gastric  space 
(Convoluta),  or  there  may  or  may  not  be  a  central 
space  (Mesostomum)  :  such  forms,  of  course,  obtain 
their  nutriment  by  intracellular  digestion.  The 
tube,  when  distinctly  formed,  may  be  simple  through- 
out, and  blind  at  the  end  opposite  the  mouth ;  or 
there  may  be  a  muscular  pharynx,  and  the  tube 
may  have  a  vent  or  anus.  The  tube  may  be 
bifurcated  in  its  hinder  part  (some  Trematoda),  or 
may  give  off  a  large  number  of  branches,  which,  as 
in  the  fluke,  ramify  through  the  body,  and  either 
end  blindly,  or  communicate  with  one  another ;  in 
the  latter  cases  the  gastric  canals  have  also  a  circu- 
latory function,  just  like  the  gastro vascular  canals  of 
the  Medusae.  (See  page  110.) 

The  Nematohelmintlies  have  the  mouth  at  one 
end  of  their  elongated  body,  and  the  anus  not  far 
from  the  opposite  end  ;  the  digestive  tube  is  perfectly 
straight,  and  is  strengthened  anteriorly  by  a  deposit 
of  chitin.  The  mouth,  which  sometimes  (Gordius) 
disappears  during  the  course  of  development,  but  not, 
curiously  enough,  until  the  worm  has  ceased  to  live  an 
endoparasitic  life,  is  only  provided  with  circumoral 
bristles  in  such  (Anguillulidae)  as  never  pass  any 
part  of  their  lives  within  other  animals.  Anteriorly 
the  tube  is  often  widened  out,  well  supplied  with 
muscles,  and  converted  into  a  sucking  apparatus. 

The  Earthworms  afford  an  example  of  how  an 
animal  may  atone  for  the  absence  of  certain  organs 
by  what  may  be  really  regarded  as  artificial  means  ; 
though  they  live  on  all  kinds  of  food,  and  especially 
on  leaves,  they  are  without  any  organ  by  means  of 
which  their  food  may  be  broken  up  ;  to  effect  this 
they  swallow  small  stones,  which,  acted  on  by  the 
contraction  of  the  muscles  in  the  walls  of  that 


chap,  iv.]       DIGESTION  IN  EARTHWORMS. 


portion  of  their  intestine  which  is 
gizzard,  are  able  to  pound  the  food 
which  has  been  taken  into  it;  the 
same  phenomenon  is  known  to  be 
observed  in  grain-eating  birds.  But 
this  is  not  the  only  method  by  means 
of  which  the  earthworm,  with  its  un- 
armed mouth,  is  able  to  act  on  the  so 
often  dry  food  on  which  it  lives  j  as 
Mr.  Darwin  pointed  out,  we  observe 
in  them  a  case  of  extra-stomachal  di- 
gestion, which,  so  far  as  is  known,  is 
unique  in  the  animal  kingdom.  Before 
proceeding  to  swallow  its  food,  the 
worm  bathes  it  in  a  fluid  secreted  by 
the  glands  of  the  mouth  ;  this  has  not 
merely  a  lubricating,  but  a  distinct 
chemical  action,  the  contents  of  the 
cells  and  the  starch  granules  being,  in 
some  observed  cases,  dissolved  out  be- 
fore the  leaves  were  taken  into  the 
mouth.  The  parts  of  the  leaves  thus 
acted  on  seemed  to  be  sucked  into  the 
mouth  by  the  action  of  the  muscular 
pharynx  (Fig.  56)  •  as  the  food  passes 
down  the  completely  straight  intestine, 
it  meets  in  the  oesophagus  with  the 
secretion  of  three  pairs  of  calciferous 
glands,  in  which  we  find  crystals  or 
concretions  of  carbonate  of  lime.  It 
would  appear  that  these  glands  are 
first  of  all  excretory  organs,  but  the 
excretion  seems  to  have  a  definite 
action  on  the  food,  and  to  prepare  it 
for  the  action  of  the  gastric  juices. 

The  secretion  of  the  cells  of  the 
intestine,  by  the  action  of  which  the 


Fig.  56.— Diagram 
of  the  Alimen- 
tary Canal  of  an 
Earthworm. 
(After  Ray  Lan- 
kester.) 

m,  Mouth ;  ph,  pha- 
rynx; ces,  oesopha- 
gus ;  eg,  calcareous 
glands  ;  cp,  crop  ;  g, 
gizzard ;  i,  intestine. 


n6    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

food  is  brought  into  condition  suitable  for  absorption, 
is  unable  to  exercise  its  activity  unless  the  food  on 
which  it  acts  is  alkaline  in  reaction  ;  in  other  words, 
its  activity  is  arrested  in  an  acid  solution.  Now,  the 
process  of  the  decay  of  leaves  is  accompanied  by  the 
formation  of  several  acids,  which  must  necessarily  be 
neutralised  before  the  digestive  fluids  can  act  on  the 


a 


Fig.  57.— Transverse  Section  of  Earthworm  to  show  the  Position  and 
Relations  of  the  Intestine. 

«,  Cuticle ;  6,  hypodernris ;  c,  layer  of  circular  muscles  :  d,  layer  of  longitu- 
dinal muscles;  i,  enteric  cavity;  m,  "green  layer";  n,  dorsal  vessel ; 
o, "  liver."  (After  Clapartde.) 


ingested  leaves;  this  neutralisation  appears  to  be 
effected  by  the  calcareous  concretions  on  which  the 
so -called  humus  acids  readily  act ;  the  result  of  their 
union  is  an  alkaline  liquid. 

Below  the  calciferous  glands  the  oesophagus  widens 
out  into  a  crop,  and  this  is  succeeded  by  a  gizzard, 
which  is  provided  with  powerful  transverse  muscles, 
and  ordinarily  contains,  as  has  been  already  stated, 
small  stones  and  grains  of  sand ;  by  the  powerful  con- 
traction of  its  muscular  walls  and  by  the  aid  of  these 


chap,  iv.]          DIGESTION  IN  ANNUL  AT  A.  117 

stones  the  gizzard  becomes  the  organ  of  the  earth- 
worm in  which  the  food  is  triturated,  or  ground  up. 
Beyond  the  gizzard  the  intestine  runs  straight  back- 
wards to  the  anus,  which  is  placed  quite  at  the  end  of 
the  body.  In  this  intestine  we  first  meet  with  a 
structure  which  will  reappear  in  other  groups,  and 
affords  us  the  first  example  of  a  method  by  which  the 
absorbing  capacity  of  the  intestine  may  be  increased 
with  the  greatest  economy  of  space.  A  transverse 
section  of  an  intestine  reveals  the  presence  of  a  fold 
which  runs  along  the  median  dorsal  line  and  projects 
•  into  the  enteric  cavity.  This  is  the  so-called  typhlo- 
sole,  or  blind  tube.  Around  the  intestine  are  a 
number  of  granular  greenish  cells  (Fig.  57;  m),  which 
become  specially  aggregated  together  on  the  dorsal 
surface  to  form  the  so-called  "  liver  "  (o) ;  the  function 
of  this  aggregation  of  cells  is  unknown,  but  it  is  un- 
doubtedly misleading  to  apply  to  it  a  term  of  such 
definite  significance  as  that  by  which  it  is  known. 
This  remark  will  apply  also  to  the  so-called  livers  of 
other  invertebrate  animals. 

We  may  easily  pass  from  the  intestinal  tract  of 
the  earthworm  to  those  of  the  other  ringed  worms. 
The  absolutely  unarmed  condition  of  the  mouth  is  not, 
of  course,  to  be  expected  in  a  blood- sucking  or  vora- 
cious form,  and  thus  it  is  that  we  find  the  leech 
provided  with  three  chitinous  "jaws,"  hardened  by  a 
little  carbonate  of  lime,  the  edges  of  which  are 
minutely  serrated,  and  which  are  provided  with  a 
special  system  of  muscles  by  means  of  which  they  are 
able  to  work  on  one  another  ;  so,  again,  one  or  more 
pairs  of  hard  chitinous  or  even  calcareous  teeth  are 
developed  in  the  free-living  marine  worms ;  these, 
which  are  generally  hooked  and  serrated  on  their  con- 
cave edge,  work  from  side  to  side..  The  earthworm  is 
enabled  to  push  its  pharynx  forwards  when  seizing 
food,  but  the  voracious  sea-worms  can  protrude  their 


u8   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

pharynx  to  some  considerable  distance  and  so  give  to 
it  the  function  of  a  proboscis. 

The  region  of  the  crop  of  the  earthworm  is,  in  the 
leech,  specially  modified  in  relation  to  the  blood-suck- 
ing habits  of  that  form  ;  from  either  side  it  gives  off 
as  many  as  eleven  tubes  or  blind  diverticula,  which 
occupy  a  very  large  proportion  of  the  body  cavity,  and 
appear  to  serve  as  strainers  of  the  watery  portion  of 
the  blood  which  is  pressed  out  through  their  walls. 
The  development  of  caeca  is  not,  however,  confined  to 
the  leech,  for  it  is  found  also  in  the  sea-mouse  (Aphro- 
dite), where  the  very  numerous  caeca  are  branched 
towards  their  free  ends ;  in  many  other  marine 
worms  the  intestine  has  a  more  or  less  sacculated 
appearance,  owing  to  the  tube  being  constricted  at  the 
points  where  the  septa  between  the  body-segments  are 
developed. 

The  Cepliyrea  contrast  strongly  with  the  Annu- 
lata  so  far  as  the  arrangement  of  their  intestine  is 
concerned,  for  this,  in  place  of  being  straight,  is 
ordinarily  coiled,  and  the  anal  opening  is  often  found 
within  the  limits  of  the  anterior  third  of  the  body. 
The  most  anterior  portion  of  the  tract  has  here,  again, 
the  function  of  a  proboscis,  and  is  sometimes  sur- 
rounded by  retractile  tentacles  ;  in  Bonellia,  a  form 
which  in  adult  life  lives  in  mud  or  shells,  the  proboscis 
is  of  great  length,  and  is  divided  into  two  lobes  at  its 
free  end ;  along  the  ventral  surface  of  this  organ  there 
runs  a  ciliated  groove  which  reaches  to  the  mouth, 
and  the  whole  apparatus  is  capable  of  being  retracted 
with  great  rapidity. 

The  Rotatoria  obtain  their  food  from  the  cur- 
rents of  water  which  are  set  in  motion  by  the  cilia  on 
their  "  wheel-organ "  or  disc ;  and  comminute  it  by 
means  of  a  system  of  hard  parts  which  is  placed  in  an 
anterior  enlargement  of  the  intestine,  and  consists 
typically  of  two  hammer-like  pieces  which  are  set 


chap,  iv.]  ROTIFERS.  119 

laterally,  and  are  caused,  by  the  contraction  of  the 
muscles  connected  with  them,  to  work  upon  two 
centrally  set  pieces,  which  may  be  regarded  as  forming 
an  anvil.  Notwithstanding  the  minuteness  of  these 
forms,  it  has  been  possible  to  form  some  idea  as  to 
the  character  of  the  secretions  of  their  digestive  cells ; 
red  monads  swallowed  by  them  exhibiting  a  bright  red 
colour  in  the  stomach,  thanks,  apparently,  to  the  acid 
reaction  of  the  gastric  juice  of  these  forms  ;  in  the 
other  parts  of  the  intestine  they  have  been  seen  to  be 
of  a  dark  or  brown-red  colour,  owing  to  the  neutral  or 
alkaline  reactions  of  the  contents  of  that  region 
(Cohn). 

The  characters  of  the  digestive  tract  of  the 
Rotifers  present  us  with  several  instructive  phe- 
nomena, for  we  find  that  in  the  males,  which  are 
always  smaller  than  the  females,  the  intestine  is 
nothing  more  than  a  solid  cord  of  cells,  while  some- 
times there  is  in  the  females  themselves  an  indication 
of  degradation  in  the  arrested  development  of  the 
terminal  portion  of  the  gut,  and  the  consequent  return 
to  the  lower  aproctoiis  condition  or  stage  in  which 
an  anus  is  wanting.  Nor  is  this  all ;  while  the  males 
of  Nematoid  worms  are  distinguished  from  the  females 
by  having  the  generative  ducts  opening  to  the  exterior 
by  a  passage  common  to  them  and  the  intestine,  the 
Rotifer  among  Yermes  presents  an  arrangement  which 
is  exceedingly  common  among  the  Vertebraba ;  that  is, 
the  possession  of  an  enlargement  or  cloaca!  chamber 
into  which  there  open  not  only  the  digestive  and 
generative  tubes,  but  also  the  canals  of  the  excretory 
system. 

The  fixed  Bryozoa  likewise  obtain  their  food  by 
means  of  the  currents  of  water  which  they  set  in. 
motion  with  the  cilia  that  cover  the  surfaces  of  their 
protrusible  tentacles;  in  a  number  the  mouth  is 
guarded  by  an  outgrowth  (epistome)  which  has  a 


120    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

singular  resemblance  to  the  foot  of  Molluscs.  (See 
page  78.)  Though  the  epistome  is  probably  a  guard 
for  the  mouth  in  most  of  the  Bryozoa  that  possess  it, 
it  is  undoubtedly  an  organ  of  locomotion  in  the  re- 
markable genus  Rhabdopleura.  The  enteric  tract  is 
folded  on  itself,  so  that  the  anus  is  always  near  the 
anterior  end ;  it  is,  indeed,  placed  either  within 
(Endoprocta)  or  without  the  circlet  of  tentacles 
(Ectoprocta). 

Among  the  Echinodermata,  where  the  mouth 
is  ordinarily  placed  in  the  centre  of  the  disc,  we 
find  that  there  are  either  no  masticatory  organs, 
as  in  the  Crinoids,  or  that  the  hard  skeletal  pieces 
are  specially  modified  in  the  region  of  the  mouth 
to  form  the  so-called  "  odontophore  "  of  starfishes,  or 
the  various  kinds  of  mouth  papillae  which  are  found 
in  the  Ophiuroids.  These  are  ordinarily  said  to  have 
a  masticatory  function,  but  their  small  size  and  feeble 
development  justifies  us  rather  in  looking  upon  them 
as  mere  filters.  In  the  regular  Echinoids,  or  those  in 
which  the  spherical  form  of  the  body  is  retained 
throughout  life,  a  very  elaborate  system  of  penta- 
merally  arranged  parts  is  developed,  the  appearance  of 
which,  en  masse,  has  given  rise  to  the  popular  term  of 
"Aristotle's  Lantern."  Each  fifth  part  of  this  lantern 
consists  of  a  hard  tooth,  bevelled  at  the  free  edge  like 
that  of  a  rabbit  or  a  rat,  so  as  to  keep  constantly  a 
sharp  free  edge  ;  this  is  supported  in  a  framework, 
and  connected  by  muscles  with  an  arched  piece 
(auricle)  developed  on  the  interior  of  the  test.  In 
Echinanthus  and  its  allies  this  "  dental  pyramid  "  is 
less  complex,  and  in  the  Spatangoids  it  has  disap- 
peared altogether ;  so  that  these  last  are  reduced  to 
living  on  such  organic  material  as  is  to  be  found  in  the 
sand,  which  they  scoop  up  by  the  aid  of  their  spout- 
shaped  mouths.  Holothurians,  likewise,  are  without 
any  special  dentary  organs,  though  the  walls  of  their 


Chap.  IV.]  ECHINODERMS.  121 

oesophagus  are  ordinarily  strengthened  by  the  deposit 
of  calcareous  plates,  which  are  sometimes  very  regu- 
larly arranged. 

The  walls  of  the  intestines  of  Echinoderms  are  in 
all  cases  remarkably  thin,  and  but  feebly  provided  with 
muscular  tissue,  a  somewhat  remarkable  arrangement, 
when  we  reflect  that  the  movement  of  food  in  their 
digestive  tract  can  be  by  no  means  aided  by  the 
pressure  of  their  body  walls  on  the  enteric  tube 
within. 

In  the  Crinoidea  the  anal  is  always  near  the  oral 
orifice,  and  is  placed  on  a  projecting  cone ;  in  Holopus, 
as  in  some  starfishes  (e.g.  Astropecteii)  and  in  all 
Ophiuroids,  the  anus  is  lost,  so  that  here  we  have  an 
example  of  the  fact  that  the  absence  of  an  aims  is  not 
always  to  be  regarded  as  a  primitive  condition. 
There  can  be  no  reasonable  doubt  that  the  Crinoids 
are  older  than  the  rest  of  the  Echinoderms,  and  it  is 
only  in  the  most  aberrant  of  these  that  we  find  an 
anus  absent.  Where  an  anus  is  present  it  is,  except 
in  Crinoids,  placed  typically  at  the  opposite  pole  of  the 
body  to  the  mouth  ;  but  in  the  irregular  Echinids  we 
find  a  most  interesting  series  in  the  way  of  modifica- 
tion ;  thus,  in  Rhyncopygus  it  is  on  the  "  back,"  but 
not  at  the  apical  pole ;  in  Echinolampas  it  is  at  the 
edge  of  the  test,  where  the  upper  passes  into  the  under 
surface  ;  while  in  Echinoneus  it  is  quite  close  to  the 
mouth,  and,  therefore,  completely  on  the  ventral 
surface. 

The  intestine  is  either  saccular,  as  in  the  aproctous 
Echinoderms,  or  spirally  coiled  as  in  Crinoids  and 
developing  starfishes,  or  looped  as  in  Holothurians ; 
in  the  proctuchous  Asteroids  and  in  some  Echinids 
it  is  provided  with  caeca,  which  in  the  former  are 
paired,  and  extend  some  way  down  the  cavity  of  each 
of  the  arms.  These  so-called  "hepatic  cseca  "  have 
been  found  to  have  on  fibrin  the  action  of  peptic 


122    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

ferments,  part  of  the  fibrin  being  converted  into 
peptones ;  and  on  starch  that  of  salivary  fluids. 
As  in  many  other  Invertebrata,  the  term  hepatic  has 
been  applied  to  regions  of  the  digestive  tract,  rather 
on  account  of  the  brown  coloration  of  these  regions, 
than  from  the  definite  experimental  knowledge  that 
their  secretions  have  in  any  way  the  functions  of 
a  human  liver.  Some  starfishes  are  capable  of  pro- 
truding the  oasophageal  portion  of  their  intestine,  and 
of  engulfing  prey,  which  they  then  draw  into  their 
bodies. 

It  cannot  be  too  much  insisted  on  that  one  of  the 
most  prominent  characteristics  of  the  Artliropoda 
is  the  conversion  of  one  or  more  pairs  of  its  appendages 
to  the  service  of  the  mouth  ;  they  become,  in  fact, 
mouth-organs  (gnathites),  and  are,  from  a  physio- 
logical point  of  view,  to  be  regarded  as  part  of  the 
digestive  apparatus. 

There  is,  perhaps,  no  investigation  which  can  be 
more  interesting  than  the  study  of  the  modifications 
undergone  by  these  parts,  whether  we  examine  a 
single  individual,  such  as  the  lobster,  with  its  six 
pairs  of  mouth  organs,  or  extend  our  survey  over 
the  whole  series  of  arthropod ous  forms  ;  in  the  one 
case  we  observe  the  modifications  undergone  by 
similarly  constituted  parts  as  they  take  on  different 
parts  in  the  duty  of  performing  a  common  function, 
and,  in  the  other,  we  see  a  multitude  of  changes,  con- 
ditioned by  differences  in  affinity  and  in  habit.  The 
remarkable  phenomena  associated  with  the  parasitic 
mode  of  life  of  some  members  of  this  phylum  will  be 
considered  later  on.  (See  page  179.) 

When  we  examine  a  lobster  or  a  crayfish,  we  find 
that  six  pairs  of  appendages  enter  into  the  service  of 
the  mouth,  and  that  in  most  of  them  we  can  make 
out  the  leading  points  in  organisation,  which  are  cha- 
racteristic of  a  "  typical  "  appendage.  (See  page  301.) 


Chap,  iv.]     MOUTH  ORGANS  OF  CRAYFISH. 


123 


There  is,  in  other  words,  a  basal  portion  and  two 
branches  more  or  less  well  developed. 

Of  all,  the  most  modified  is  the  first  of  the  six, "or 
inaudible,  for  here  the  basal  portion  is  very  strong, 


Fig.  68.— Mouth  Organs  of  the  Crayfish. 

•*,  Mandible ;  B,  first  maxilla ;  c,  second  maxilla  ;  bp,  basipodite  ;  en,  endopodite ; 
cxp,  coxopodite  ;  p,  palp  of  mandible ;  eg,  scaphognathite.    (After  Huxley.) 

and  gives  rise  to  two  toothed  ridges  ;  of  these  the 
lower  projects  farther  than  the  upper,  and  has  a  more 
sharply  serrated  edge ;  of  the  two  branches  of  a 
typical  appendage,  the  endopodite  is  alone  developed, 
and  that  feebly,  for  it  consists  only  of  three  compara- 
tively short  joints  (palp). 


124    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

From  the  appendage  next  behind,  or  first  pair 
of  maxillae,  the  outer  branch,  or  exopodite,  is 

still  absent,  but  the  basal  portion  is  well  developed, 
though  not  so  stout  or  so  strong  as  in  the  case  of  the 
mandible ;  both  its  joints  are  flattened  out  and  pro- 
vided with  a  number  of  bristles,  which  are  also  present, 
though  less  numerous  and  not  so  strong  on  the  un- 
jointed  piece  which  represents  the  eiidopodite.  So 
far  as  the  digestive  process  is  concerned,  the  second 
pair  of  maxillee  are  still  chiefly  represented  by 
the  two  basal  joints  of  the  typical  appendage,  the 
endopodite  being  still  small  and  undivided,  while  the 
exopodite,  though  developed,  has  duties  to  perform 
in  relation  to  the  respiratory  organs.  (See  page  225.) 
Behind  the  maxillse  we  find  three  pairs  of  maxil- 
lipedes,  or  foot-jaws,  the  most  posterior  of  which  is 
the  largest,  and  in  a  state  of  repose  covers  over  the 
five  pairs  of  mouth  organs  that  lie  in  front  of  it.  In 
the  two  more  posterior  pairs  we  do  not  observe  that 
increase  in  size  or  flattening  out  of  the  basal  portion 
which  we  saw  in  the  maxillae ;  but  in  the  first 
maxillipede  the  most  important  part  is  taken  by 
the  two  lamellar  joints,  of  which  that  portion  is 
composed,  while  the  endopodite  consists  only  of  two 
in  the  place  of  the  five  distinct  joints  which  are  found 
in  the  succeeding  pairs. 

All  these  appendages  are  <3O  articulated  to  the 
walls  of  the  body  that  they  work  on  one  another  from 
side  to  side ;  it  is  clear  that  they  can  only  cut  or  tear 
the  food  on  which  the  great  forceps  have  already 
seized,  and,  for  the  purposes  of  digestion,  the  food  has 
to  undergo  a  further  comminution,  comparable  in  a 
sense  to  that  which  is  performed  by  the  grinding,  as 
distinguished  from  the  cutting  teeth  of  man. 

The  mouth,  which  is  a  narrow  elongated  slit,  leads 
by  a  short  wide  gullet  into  a  capacious  stomach, 
divided  into  an  anterior  and  a  posterior  chamber,  and 


chap,  iv.]      GASTRIC  MILL  OF  CRAYFISH. 


125 


separated  from  the  intestine,  which  lies  behind  it,  by  a 
filtering  apparatus  of  valves  and  bristles.      Within 


Fig.  59.— The  Parts  of  the  "  Gastric  Mill  "  of  the  Crayfish  in  situ  (A), 
and  disarticulated  (B). 

c,  Cardiac  ossicle;  pc,  pterocardiac  ossicle;  zc,  zygrocardiac  ossicle ;  It,  lateral  tooth  ; 
p,  pylpric  ossicle;  uc,  urocardiac  ossicle  with  (&)  accessory  tooth;  pp,  pre- 
pyloric  ossicle  with  (mt)  median  tooth.  (After  T.  J.  Parker.) 

this  stomach  there  is  developed  a  hexagonal  frame- 
work of  calcareous  arid  chitinous  pieces,  some  of  which 
are  provided  with  powerful  grinding  teeth.  The  fore 
(Fig.  59  ;  c)  and  hind  (p)  sides  of  the  hexagon  give 
off,  the  one  forwards  and  the  other  backwards,  an 
elongated  ossicle  (uc,  pp),  each  so  placed  in  relation  to 


126    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  other  that,  when  at  rest,  the  two  make  an  open 
angle  towards  the  dorsal  aspect,  while  the  more  pos- 
terior has  at  its  lower  edge  a  strong  tooth  (mt)  which 
takes  a  backward  direction.  To  the  fore  and  hind 
bars  of  the  hexagonal  framework  there  are  attached 
strong  muscles,  which,  by  contracting,  draw  these  two 
bars  away  from  one  another.  This  separation  of  the 
terminal  naturally  requires  an  approximation  of  the 
lateral  faces  (pc,  zc),  two  of  which  bear  strong  teeth. 
While  these  teeth  are  thus  brought  closer  to  one 
another,  the  angulated  bar  which  connects  the  fore  and 
hind  pieces  of  the  hexagon  becomes  straightened  out ; 
the  result  of  this  straightening  is  seen  in  the  downward 
and  forward  movement  of  the  tooth  which  is  developed 
on  the  hinder  median  bar  (mt)  and  which  is  thereby 
brought  into  closer  relation  with  the  approximating 
teeth  on  the  side-pieces.  (See  Fig.  59.)  This  elaborate 
"  gastric  mill "  must  break  up  the  food-masses  taken 
in  by  the  crayfish ;  but,  as  if  this  were  not  enough, 
the  hinder  part  of  the  so-called  pyloric  region  of  the 
stomach  is  provided  with  cushions  covered  with  hairs, 
and  longitudinal  ridges  with  still  finer  hairs,  which 
form  a  most  efficient  filtering  apparatus.  This  may, 
from  a  physiological  point  of  view,  be  compared  with 
the  sieve  of  hairs  which  lies  at  the  entrance  to  the  in- 
testine of  the  fish-eating  bird,  the  darter  (Plotus). 

As  far  as  this  filter  the  whole  of  the  enteric  tract 
will  be  found  to  have  its  inner  face  lined  with  chitin  ; 
the  next  succeeding  portion,  which  forms  the  com- 
mencement of  the  delicate  "  intestine,"  has  no  such 
internal  layer  ;  but  this  in  the  crayfish,  though  not  in 
the  lobster,  is  quite  short.  On  it  there  follows  the 
remainder  of  the  "  intestine,"  and  this  will  be  found 
to  be  again  lined  with  chitin. 

When  we  come  to  ask  ourselves  why  so  much,  yet 
not  all,  of  the  enteric  tract  of  the  crayfish  is  thus  lined 
by  the  same  dense  body  as  that  which  forms  the  outer 


Chap,  iv.]    MOUTH  ORGANS  OF  TRACHEATA.         127 

covering  of  a  crayfish's  body,  we  are  compelled  to 
turn  to  the  history  of  development  for  an  explanation. 
When  we  do  this,  we  find  that  the  epiblastic  infoldings, 
which  form  respectively  the  stomodoeum  and  the 
proctodoemn,  are  carried  very  far  inwards,  and  that 
only  a  small  portion  of  the  archenteron,  or  region 
primitively  lined  by  hypoblast,  remains  in  the  adult 
organisation.  The  developing  lobster,  as  compared 
with  the  developing  crayfish,  has  a  much  shorter  proc- 
todeal  invagination. 

While  it  is  absolutely  true  of  all  the  animals  here 
spoken  of  as  Arthropoda  that  some  of  their  appendages 
are  converted  into  mouth  organs  or  gnathites,  the 
number  is  by  no  means  always  so  large  or  the  arrange- 
ments so  complicated  as  those  which  we  have  just 
found  to  obtain  in  the  crayfish.  In  Peripatus,  for 
example,  one  pair  only  of  appendages  are  modified  to 
serve  as  jaws,  which  have  the  special  function  of 
cutting  blades.  In  the  Scorpion,  where  there  are  no 
appendages  in  front  of  the  mouth,  there  are  only  two 
pairs  specially  adapted  to  the  service  of  the  mouth, 
and  these  have  their  free  ends  pincer-shaped,  and  not 
converted  into  cutting  or  biting  organs  ;  this  arrange- 
ment will  be  the  more  clearly  understood  when  one 
remembers  that  these  animals  suck  the  juices  rather 
than  eat  the  tissues  of  their  prey. 

The  differences  between  the  Chilopodous  and 
Chilognathous  Myriopoda  allow  us  to  say  little 
that  is  true  of  them  both  ;  in  both,  however,  we  see 
here,  as  in  other  parts  of  their  organisation,  characteris- 
tics of  a  less  high  degree  of  differentiation  than  those 
that  obtain  in  the  crayfish,  on  the  one  hand,  or  the 
cockroach  on  the  other ;  there  are  two  or  three  pairs 
of  gnathites,  and  these  are  always  jointed.  One  is 
often  converted  into  u  poison  gland  in  the  Chilopoda, 
and  in  them  also,  as  in  the  scorpion,  the  basal  por- 
tion of  some  of  the  succeeding  pairs  of  appendages 


128    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

surround  the  orifice,  and  aid  the  work  of  the 
mouth. 

In  the  lower  Crustacea  (Entomostraca)  we  never 
find  more  than  three  pairs  of  appendages  converted 
into  gnathites,  and  these  are,  in  a  general  way,  com- 
parable to  the  mandibles  and  maxillae  of  the  crayfish. 

The  true  Insects  or  Arthropoda  Hexapoda  have, 
likewise,  three  pairs  of  mouth  appendages,  and  these, 
again,  are  known  as  mandibles  and  maxillae ;  but  the 
mandible  is  never  provided  with  the  three-jointed 
"  palp,"  which  is  found  in  the  crayfish. 

No  series  of  structural  changes  in  relation  to  the 
different  modes  of  taking  in  food  is  more  interesting 
than  the  really  remarkable  variations  which  are  found 
in  the  size  and  shape  of  the  gnathites  of  insects,  and 
nowhere,  perhaps,  do  we  see  more  distinctly  the  in- 
fluences of  those  two  prime  factors  in  organic  evolution, 
heredity  and  adaptation. 

As  Meynert  has  pointed  out,  we  find  in  winged 
insects  two  chief  types  of  mouth  organs  ;  in  some  the 
mandibles  are  hinged  on  to  the  sides  of  the  head,  and 
the  first  pair  of  maxillse  have  a  less  perfect  articu- 
lation ;  sometimes,  indeed,  the  latter  merely  slide  on 
the  sides  of  the  hard  parts  which  bound  the  mouth  ; 
in  others  the  mandibles  and  maxillse  are  not  arti- 
culated, but  can  be  withdrawn  inwards,  or  protruded 
outwards. 

In  the  former  the  articulation  is  such  that  the 
gnafchites  work  from  side  to  side  and  are  fit  to  act  as 
cutting:  or  biting  organs  ;  in  the  latter  they  can  be 
pushed  into  an  object  or  laid  side  to  side,  so  that  they 
form  stabbing  or  sucking  parts. 

It  is  of  supreme  interest  to  observe  that  among 
the  members  of  the  lower  grade  of  insects,  or  that  in 
which  wings  are  never  developed  (Aptera),  the 
mouth  organs  sometimes  (Campodea)  remain  in  an 
undifierentiated  or  generalised  condition ;  though  not 


chap,  iv.]     MOUTH  ORGANS  OF  INSECTS.  129 

articulated  to  the  sides  of  the  head,  they  can  be 
moved  by  muscles  from  side  to  side,  while,  thanks  to 
the  absence  of  the  articulation,  they  can  be  pushed  out 
or  drawn  in ;  they  are,  in  fine,  capable  of  acting  either 
as  cutting  or  as  stabbing  organs,  and  it  is  in  them, 
therefore,  that  we  must  look  for  that  indifferent  ar- 
rangement from  which  both  the  mandifoiilate  and 
haustellate  type  of  mouth  organs  have  had  their 
origin. 

The  former  is  well  seen  in  the  ancient  group  of 
the  Orthoptera,  and  is  easily  demonstrated  by  the 
familiar  example  of  the  cockroach.  In  this  form  it  is 
quite  easy  to  recognise  the  three  pairs  of  gnathites 
which,  in  insects,  form  those  organs  of  the  mouth 
which  are  derived  from  modified  appendages,  the  one 
pair  of  mandibles,  and  the  two  pairs  of  maxillse. 

In  front  of  the  mouth  there  is  an  upper  lip  or 
labriim,  which  has  the  form  of  a  movable  flap ; 
behind  it  lie  the  mandibles  (Fig.  60 ;  md),  modified 
appendages,  of  which  no  part  other  than  the  basal 
remains,  all  signs  of  a  palp  having  completely  dis- 
appeared; these  work  from  side  to  side,  and  have 
their  inner  edge  strongly  toothed,  so  that  they  act  as 
efficient  biting  organs. 

Behind  these  we  find  the  first  pair  of  maxillae, 
organs  of  some  size  and  complexity ;  the  basal  piece 
or  cardo  (ca)  is  articulated  to  the  head,  and  has, 
at  right  angles  to  its  long  axis,  the  second  joint  or 
stipes  (st) ;  this  can  move  in  a  lateral  direction,  and 
is  continued  forwards  into  a  soft  galea  (go),  and  an 
internally  placed  lacinia  (la),  the  inner  edge  of 
which  is  toothed,  though  not  so  strongly  as  the 
mandibles.  Attached  to  the  outer  side  of  the  stipes 
is  the  so-called  palp. 

The  second  pair  of  maxillse  are  still  further  modi- 
fied, presenting  as  they  do  confluence  of  the  basal 
portions,  which  in  most  air-breathing  Arthropods  is 
J— 16 


130  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

still  more  complete.  The  result  of  this  fusion  is  the 
formation  of  a  median  piece,  which  is  incompletely 
divided  into  two,  forming  the  mentiim  (m),  and 
sub  in  en  tii  an  ;  these,  with  the  anteriorly  lying 


ccl 


Fig.  60.— Mouth-parts  of  the  Cockroach. 

m,  Mentum  ;  sm,    submentum  ;  K,  ligula  ;  py,  paraglossa  ;  pp,  palp  of  second 
maxilla ;  md,  mandibles ;  ca,  cardo  ;  at,  Btlpes  ;  ja,  galea  ;  Ja,  lacmia  ;  p,  palp. 

ligiila  (li)  and  paraglossa  (pg),  make  up  the  part 
which  by  entomologists  is  most  unfortunately  spoken 
of  as  the  lower  lip  or  labium;  unfortunately,  be- 
cause it  is  formed  from  the  modification  of  the 
proximal  parts  of  an  appendage,  and  is  not  strictly 
comparable  to  the  labrum  or  upper  lip,  which  is  a 
part  of  the  exo-skeleton  of  the  head.  The  palp  (pp) 
of  the  second  pair  of  maxillae  is  smaller  than  that  of 
the  first 


Chap,  iv.]      MOUTH  ORGANS  OF  INSECTS,  131 

The    mouth    organs    of    the    Neuroptera    are 

strictly  comparable  to  those  of  the  Orthoptera ;  but 
we  see  an  advance  in  the  fusion  of  the  lateral  halves 
of  the  labium,  while  the  biting  mandibles  are  grooved 
on  their  inner  face,  and  the  first  pair  of  maxillae  are 
slender,  and  are  so  arranged  as  to  close  the  groove, 
and  to  give  rise  to  a  pair  of  organs  which  serve  as 
tubes  for  the  passage  of  the  juices  of  the  prey  which 
they  have  first  bitten. 

In  the  allied  Trichoptera  (caddis  flies)  the 
mandibles  are  reduced  to  membranous  rudiments,  and 
the  maxillae  and  labium  are  closely  united,  and  at 
their  base  come  to  be  tubular  in  form. 

In  the  Coleoptera  (beetles)  the  biting  powers  of 
the  mandibles  seem  to  reach  the  maximum  of  their 
development,  and  the  labium  has  the  mentum  and 
submentum  united  into  single  piece. 

In  the  Hymenoptera  (bees  and  wasps)  the 
mandibles  still  retain  their  biting  function,  but  the 
maxillae  are  modified  to  serve  as  licking  and  sucking 
organs  ;  the  ligula  and  the  first  pair  of  maxillae  are 
greatly  elongated,  and  the  latter  apply  themselves  to 
the  sides  of  the  former,  giving  rise  with  it  to  a 
tubular  apparatus,  which  comes  into  play  after  the 
elongated  ligula  (or  its  accessory  piece)  has  licked  up 
the  honey  on  which  their  possessor  depends. 

The  conversion  of  the  mouth  parts  into  a  sucking 
organ  is  most  completely  seen  in  the  butterfly 
(Lepidoptera) ;  the  mandibles  are  reduced  to 
mere  rudiments,  and  the  first  pair  of  maxillae 
are  greatly  elongated ;  the  inner  face  of  each  of 
these  last  is  deeply  grooved,  and  the  edge  of  the 
grooves  minutely  denticulated  in  such  a  manner  that, 
when  one  maxilla  is  applied  to  its  fellow  of  the 
opposite  side,  it  combines  with  it  to  form  a  closed 
tube ;  the  labium  is  reduced,  and  its  palps  are  often 
very  small  or  evanescent.  The  sucking  tube  may 


132   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


become  of  considerable  length  when  the  Lepidopteron 
feeds  on  the  honey  of  plants,  such  as  orchids,  in 
which  the  nectaries  are  at  a  considerable  distance 
from  the  outer  edges  of  the  flowers ;  in,  for  example, 
Amphonyx,  one  of  the  Sphingidse,  the  proboscis  is 
nine  and  a  quarter  inches  long,  or  about  three  times 
the  length  of  the  animal's  body.  In  some  Lepidoptera 
the  proboscis  is  enlarged  to  pierce  vegetable  tissues, 
and,  as  in  the  orange-sucking 
Ophideres,  it  has  externally  the 
form  and  function  of  a  bayonet- 
shaped  saw  (F.  Darwin). 

In  the  blood-  or  juice-sucking 
Hemiptera  (bugs,  aphides)  not 
only  the  mandibles  but  also 
the  first  pair  of  maxillae  are  re- 
duced to  fine  setiform  processes, 
vvhich,  being  moved  by  muscles, 
are  enabled  to  serve  as  stabbing 
organs  ;  they  are  ensheathed  in 
the  elongated  labium  (rostrum) 
the  sides  of  which  curve  up- 
wards in  such  a  way  as  to 
produce  a  sucking-tube  (Fig.  61). 
In  the  Diptera  (or  flies 
and  fleas),  what  were  bristles  in 
the  bug  now  form  sharp,  cut- 
ting, lance c-like  organs,  and  the 
second  pair  of  maxillse  again 
form  the  suctorial  tube  ;  in  some 
cases  (Pangonia)  the  proboscis 
is  more  than  twice  as  long  as  the  body. 

Allied  to  various  orders  of  insects  are  forms  which, 
in  correlation  with  their  modes  of  life,  have  their 
gnathites  still  more  considerably  altered  from  the  Or- 
thopterous  type ;  thus,  among  the  white  ants  (Termi- 
tidse)  the  mandibles  are  functional  in  the  so-called 


Fig.  61.— Mouth  Orgaii  of 
Nepa. 

md,  Mandible ;  mx,  first  pair 
of  maxilla;  ma/,  second 
pair  (labium);  li,  ligula. 
(After  Savigny.) 


Chap,  iv.]  ENTER  ON  OF  INSECTS.  133 

soldiers,  but  reduced  in  the  workers ;  the  Ephe- 
meridse  or  day-flies,  which  want  to  eat  no  food  in  the 
adult  stage,  have  the  gnathites  almost  completely 
aborted.  The  Mallophoga,  or  so-called  "  Mandibulate 
lice,"  which  are  found  on  the  skin  of  birds  and  mam- 
mals, and  feed  on  their  feathers  and  hairs,  have  the 
mandibles  hooked  and  the  maxillae  small. 

Like  the  crayfish,  the  cockroach  has  a  large 
portion  of  the  anterior  region  of  its  digestive  tract 
lined  with  chitin,  and,  like  that  form,  it  has  also  a 
considerable  portion  of  the  hinder  region  formed  by 
the  proctodeal  invagination.  The  chitinous  layer 
extends  through  the  funnel-shaped  pharynx,  the 
narrower  03sophagus,  the  crop-like  enlargement,  and 
the  proventriculus ;  the  last  has  the  form  of  a  trun- 
cated cone,  and  its  walls  are  thick  and  well-provided 
with  muscles;  its  internal  lining  is  raised  up  into 
ridges  which  serve  as  teeth,  and  between  these  ridges 
there  are  pouches.  The  next  succeeding  portion  has 
no  chitinous  lining,  and  its  anterior  end  has  connected 
with  it  eight  blind  prolongations  (the  so-called  pyloric 
caeca),  which  are  not  all  of  the  same  length,  and 
which  vary  in  size  according  to  the  periods  of  digestive 
activity ;  it  is,  apparently,  in  this  cavity  that  the 
food  undergoes  the  changes  which  convert  it  into 
.  chyle,  and  the  caeca  are  only  to  be  regarded  as  out- 
growths which  increase  the  capacity  of  the  ventriculus. 
The  intestine  behind  is  lined  throughout  with  chitin, 
and  the  smaller  is  separated  from  the  wider  portion 
by  a  valve  ;  the  whole  tract  ends  in  a  terminal  anus. 

As  may  well  be  supposed,  the  different  parts  of 
the  digestive  tract  present  very  different  characters 
in  the  various  groups  of  insects  •  in  the  mandi- 
bulate  forms  (Neuroptera,  Coleoptera)  the  stomach 
is  provided  with  a  series  of  more  or  less  powerful 
chitinous  ridges,  by  means  of  which  the  food  is 
comminuted;  in  the  sucking  insects  the  gizzard 


134  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

is  aborted,  but  the  crop  is  swollen  out  into  a  simple 
sac  (bees),  or  into  two  hemispherical  sacs  (blowfly), 
or  its  attached  portion  forms  a  short  narrow  tube 
and  its  free  part  a  swollen  bladder-like  enlarge- 
ment (butterfly).  This  organ  may  extend  far  back 
into  the  abdomen,  and,  as  it  has  thin  walls  and  no 
muscular  attachment  to  the  body  wall,  its  size  is 
probably  increased  and  diminished  by  the  contractions 
of  the  hinder  parts  of  the  body  ;  this  so-called  "  suck- 
ing stomach  "  appears  to  act  as  a  reservoir  for  food  in 
the  Diptera. 

At  the  anterior  end  of  the  tract  there  open  the 
ducts  of  the  salivary  glands,  which  are  ordinarily 
developed  in  insects,  but  best  seen  in  the  haustellate 
forms ;  they  vary  greatly  in  form  and  size,  and  are 
by  no  means  always  confined  to  the  function  of 
digestive  glands,  as  the  mosquito,  the  bug,  or  the 
tse-tse  fly  are  sufficient  to  bear  witness.  Many  larvae 
have  well-developed  glands  which  open  just  behind  the 
mouth,  and  which  secrete  a  body  which  in  air  hardens 
into  a  fine  silky  thread.  Glands  are  often  developed  in 
the  walls  of  the  rectum  or  large  intestine,  and  have  a 
secretion  which  is  frequently  of  a  pungent,  if  not  of  a 
disagreeable,  odour.  The  Malpighian  vessels  which 
are  connected  with  the  hinder  portion  of  the  tract 
and  open  into  it  are  not  digestive  glands,  but  organs 
of  renal  excretion.  (See  page  256.) 

We  find  a  very  different  arrangement  of  mouth 
organs  in  the  Mollusca  to  that  which  we  have  just 
been  studying  in  the  Arthropoda  ;  the  great  majority 
are  without  any  seizing  organs  of  any  kind,  and  the 
lowest,  the  Lamellibranchiata,  have  no  means  by 
which  they  can  comminute  their  food ;  they  live, 
therefore,  on  the  minute  organisms  which  are  brought 
to  them  with  the  water  of  respiration,  and  which  are 
felt  for  and  guided  to  the  mouth  by  the  blunt  "labial 
tentacles  "  that  lie  on  either  side  of  it. 


Chap,  iv.]       ODONTOPHORE  OF  MOLLUSCA.  135 

The  rest  of  the  Mollusca,  or  Cephalophora,  are 
provided  with  a  rasping  organ,  which  lies  011  the  floor 
of  the  pharynx,  the  odontophore.  But,  anteriorly 
to  this,  and  at  the  edge  of  the  mouth,  there  are  one 
or  more  horny  plates,  with  a  sharp  cutting  edge;  these 
are  best  developed  in  the  cuttle-fishes,  where  they 
have  the  appearance  of  a  parrot's  beak  turned  upside 
down ;  in  the  nautilus  these  beak-like  plates  are 
calcined. 

The  characteristic  organ  of  the  digestive  system  of 
such  Mollusca  as  have  not  suffered  degeneration  of 
the  head  is  the  just-mentioned  odontophore.  This 
consists  essentially  of  an  overlying  chitinous  sheet, 
the  surface  of  which  is  produced  into  a  variable 
number  of  more  or  less  sharp  processes,  the  so-called 
teeth ;  this,  then,  is  a  rasping  organ,  or  radii  I  si. 
Underlying  are  connective  and  muscular  fibres,  and 
supports  are  afforded  for  it  by  the  development 
beneath  of  masses  of  cartilage  ;  as  muscles  are  inserted 
into  the  anterior  and  posterior  faces  of  these  cartila- 
ginous supports,  it  is  clear  that,  by  their  alternate 
contraction  and  relaxation,  they  will  draw  the  radula 
backwards  and  forwards.  The  whole  apparatus  is 
developed  in  a  blind  diverticulum,  lying  on  the 
ventral  surface  of  the  cavity  of  the  mouth.  The 
teeth,  which  may  be  very  variously  arranged,  are 
greatly  strengthened  by  the  deposit  of  silica ;  and  as 
they  and  the  chitinous  sheet  are  worn  away  they  are 
replaced  by  the  hinder  part  of  the  radula,  which 
passes  forwards  on  its  bed ;  the  replacement  of  the 
effete  parts  being  effected,  in  other  words,  in  a  way 
comparable  to  that  of  the  human  nail.  The  radula  is 
ordinarily  divisible  into  a  central  piece,  with  a  lateral 
piece  on  either  side ;  the  teeth  on  the  former  are 
spoken  of  as  the  racliidiaii  teeth,  and  those  on  the 
lateral  pieces  as  the  iincini.  The  arrangement  of 
these  teeth  varies  very  greatly,  and,  for  the  purpose 


136  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

of  succinctly  stating  their  numbers  and  positions,  the 
following  method  of  formulation  is  used. 

The  central  teeth  of  the  rachidian  series  are 
denominated  by  the  sign  1,  when  present,  and  0  when 
absent;  the  admedian  teeth  by  the  signs  1,2,3..., 
according  to  the  number  present ;  while  the  lateral 
teeth  are  noted  by  the  sign  0  repeated  as  often  as 
there  are  lateral  teeth  on  either  side ;  when  the 
number  of  admedian  or  of  lateral  teeth  is  very  large, 
the  sign  x  is  used  in  place  of  1,  2,  3. .. ,  or  0  repeated. 
For  example,  when,  as  in  ^Eolis,  there  are  no  lateral 
teeth,  we  write  the  formula  0.1.0;  that  of  Amphis- 
phyra,  is  1  .  1 .  1  ;  that  of  Aplysia,  13 .  1 .  13  ;  and  that 
of  Oncidium,  54.  1 .  54  (Woodward). 

The  whole  mass  of  the  odontophore  may  be  of 
considerable  size,  and,  in  the  limpet,  the  radula  is  two 
or  three  times  the  length  of  the  body;  the  number 
of  separate  teeth  may  be  very  great,  as  among 
the  snails,  where  167  transverse  rows  of  135  teeth 
each  will  give  some  twenty  thousand  teeth ;  in  some 
species  of  Helicidae,  the  aggregate  exceeds  thirty- 
nine  thousand  (39,596). 

The  teeth  are  sometimes  large  and  hooked  ;  some- 
times conical  and  upstanding;  when  the  rachidian 
teeth  are,  as  sometimes  happens,  absent,  another  part 
of  the  digestive  tract  may,  as  in  the  Bullidae,  be  pro- 
vided with  calcareous  plates  which  replace  them 
functionally.  In  a  few  (e.g.  Rhodope)  the  odonto- 
phore is  lost. 

In  a  number  of  cases,  the  muscles  that  move  the 
radula  are  not  confined  to  those  that  are  inserted  into 
the  supporting  cartilages,  but  there  are  others  that 
pass  to  the  walls  of  the  head  ;  the  contraction  of 
these  is  the  cause  of  the  licking  movement  which  a 
protruded  radula  may  be  often  seen  to  perform. 

In  some,  especially  slugs  and  snails,  a  hard  horny 
plate  is  developed  on  the  roof  of  the  mouth  cavity, 


Chap,  iv.j          ENTERON  OF  MOLLUSC  A.  137 

and  aids  the  radula  in  its  work  of  trituration,  just  as 
the  hard  pad  which  takes  the  place  of  the  upper  in- 
cisors of  the  sheep  serves  as  a  resistent  structure 
against  which  the  lower  incisors  may  bite. 

At  the  sides  of  the  anterior  portion  of  the  digestive 
tract  glands  of  various  forms  are  ordinarily  found ; 
these  are  known  as  salivary  glands ;  but  the  inappro- 
priateness  of  the  name  is  not  only  obvious  from  the 
observed  fact  that  in  the  slug  the  secretion  of  these 
glands  has  no  influence  on  starch,  but  is  made  the 
more  striking  so  soon  as  we  know  that,  in  several 
genera,  the  secretion  of  these  glands  contains  a  com- 
paratively large  amount  (nearly  three  per  cent,  in 
Dolium)  of  free  sulphuric,  and  a  smaller  quantity  of 
hydrochloric,  acid.  Further,  we  have  to  note  that 
these  buccal  glands  are  found  in  marine  as  well  as  in 
terrestrial  forms,  whereas  among  the  vertebrata  the 
salivary  glands  are  only  well  developed  in  terrestrial 
forms. 

The  intestine  is  considerably  coiled ;  the  oasopha- 
geal  portion  is  sometimes  produced  into  a  "  crop,"  as 
in  Lymnseus  or  Octopus  \  the  succeeding  portion  may 
be  simple,  and  have  its  walls  thin  or  muscular,  or  it 
may  be  broken  up  into  several  chambers ;  in  Scyllsea 
it  is  armed  internally  with  horny  cutting  blades,  and 
in  Aplysia  with  blunt  horny  spines,  behind  which  is 
an  armature  of  sharp  hooks.  It  is  only  behind  such 
gizzard-like  enlargements  that  the  digestive  ferments 
are  secreted.  The  anal  orifice  is,  in  those  Cephalo- 
phora  that  have  lost  their  original  bilateral  symmetry, 
brought  far  forwards,  and  situated  near  the  mouth, 
or  is  placed  at  the  side  of  the  body.  Ca3cal  pouches 
or  tubes  are  developed  in  various  ways  along  the  tract 
of  the  intestine,  and  some  of  them  become  charged 
with  dark-coloured  cells,  and  have  been  regarded  as 
forming  a  "  liver ;  "  there  is,  however,  no  reason  for 
associating  with  these  structures  the  functions  of  the 


138  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

similarly  named  part  in  the  Yertebrata  ;  and,  indeed, 
where  best  studied,  they  have  been  found  to  have 
rather  the  function  of  the  pancreas.  In  the  dibran- 
chiate  Cephalopoda  a  rectal  caecum  secretes  an  inky 
fluid,  which  was  formerly  used  for  writing  and  for 
the  manufacture  of  sepia ;  this  is  the  so-called  ink- 
bag.  The  secretions  of  the  Octopus  have  been  found 
to  be  all  acid. 

In  the  Lamellibranchiata  (or  Acephala)  the 
odontophore  is  completely  absent ;  the  intestinal  tract 
is  comparatively  simple,  but  varies  in  the  extent  of 
its  convolutions ;  in  its  walls,  or  in  an  appended 
caecum,  is  the  so-called  crystalline  style  or  stalk,  a 
transparent  rod-like  structure  of  unknown  function. 
Its  absorbent  surface  is  sometimes  increased  by  the 
development  of  a  typhlosole,  as  in  the  earthworm, 
and  the  terminal  portion  very  frequently  passes 
through  the  dorsally-placed  heart. 

In  all  Chordata  we  observe  that,  as  also  in 
Balanoglossus,  the  anterior  posterior  of  the  diges- 
tive tract  is  primarily  divisible  into  an  upper  and  a 
lower  portion,  one  of  which  serves  as  the  means  of 
passage  for  the  water  of  respiration,  and  the  other  as 
the  food  passage.  Postponing  for  the  moment  (see 
page  231)  the  consideration  of  the  former,  and  insist- 
ing only  on  the  significance  of  this  arrangement  as  a 
leading  point  in  the  morphology  of  the  Chordata, 
we  observe  that  in  the  Tunicata  the  exclusively 
nutrient  region  of  the  enteric  tract  commences  at 
the  bottom  of  the  respiratory  part  by  a  rounded 
or  funnel-shaped  opening;  the  tube,  which  varies  in 
calibre  in  different  parts,  is  often  looped,  and  in 
such  cases  the  anus  comes  to  lie  not  far  from  the 
mouth. 

Among  the  Cliordata  we  find  very  simple 
arrangements  of  the  digestive  tract  in  the  Cepha- 
locliordata ;  the  mouth  of  the  Lancelet  is  placed 


Chap,  iv.]  ENTER  ON  OF  CHORDATA.  139 

on  the  ventral  surface  of  the  body,  not  far  from  the 
anterior  end,  is  over-hung  by  a  hood,  and  supported 
by  cartilaginous  bars,  which  bear  ciliated  cirri,  the 
gill-like  appearance  of  which  gained  for  the  animal 
the  misleading  name  of  Branchiostoma.  As  these 
cirri  are  moved  by  muscles  they  are  enabled  to  direct 
food  to  the  mouth,  and  to  serve  as  a  filter  against  the 
entrance  of  sand  and  other  useless  or  dangerous 
bodies.  As  in  the  rest  of  the  lower  Chordata,  this 
mouth  serves  as  the  orifice  of  entrance  for  the  water 
of  respiration,  which  makes  its  way  to  the  exterior 
through  numerous  spaces  in  the  wall  of  the  more  an- 
terior region  of  the  digestive  tract.  The  part  of  the 
tract  behind  the  gill  chamber  is  of  some  width,  and 
gradually  narrows  as  it  approaches  the  anus,  which 
is  situated  on  the  ventral  surface  not  far  from  the 
hinder  end  of  the  body,  At  its  anterior  end  it  gives 
off  a  forwardly  directed  short  blind  process,  which  is 
known  as  the  liver.  As  in  the  Nemertinea,  the 
enteric  epithelium  is  ciliated.  In  the  Uroclior- 
clata  a  large  part  of  the  anterior  region  of  the 
enteron  is  again  converted  into  a  respiratory  cham- 
ber ;  and  it  is  the  succeeding  portion  only  that  is 
limited  to  the  duties  of  a  digestive  apparatus.  The 
tube  varies  in  width  in  different  regions  and  is 
ordinarily  coiled  on  itself,  so  that  the  anal  is  not  far 
from  the  oral  orifice  ;  the  food  passes  into  it  along 
a  groove  which  lies  on  the  ventral  surface  of  the 
respiratory  chamber,  the  sides  of  which  are  ciliated, 
and  the  cells  of  which  secrete  a  mucous  substance 
which  entangles  the  food  and  carries  it  into  the 
03sophagus.  The  anterior  orifice  of  the  oesophagus 
is  generally  funnel-shaped,  and  provided  with  cilia ; 
the  succeeding  portion  has  a  diverticulum,  which  is 
spoken  of  as  the  liver,  and  it  may  be  further  provided 
with  other  glandular  organs.  In  some  cases  the 
digestive  tube  is  coiled  into  a  closely  compacted 


140  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

\  mass,  as  in  the  Salpidse,  where  it  forms  the  so-called 
"  nucleus." 

In  those  Vertebrates  that  breathe  by  gills  the 
water  of  respiration  enters  by  the  mouth  ;  in  the  air- 
breathing  forms  the  air  enters  the  mouth  by  the  nostrils, 
so  that  in  their  case  also  the  most  anterior  portion  of 
the  digestive  tract  serves  as  an  ante-chamber  to  the 
respiratory  organs.  Leaving  these  functions  aside 
for  the  moment  (see  page  231)  and  confining  ourselves 
to  the  mouth  as  a  part  of  the  digestive  apparatus,  we 
observe  that  it  is  rounded  in  shape  in  the  lowest, 
the  Cyclostomata  (lampreys  and  hags),  and  merely 
supported  by  cartilages  ;  in  all  the  rest  it  is  more  or 
less  slit-like,  and  a  pair  of  branchial  arches  give  rise 
to  jaws  (Gnathostomata).  These  jaws  are  either 
covered  by  connective  tissue,  or  horny  plates  (tor- 
toises, birds,  monotremata),  or,  as  in  all  Mammals, 
except  the  lowest  and  the  whales,  they  are  guarded  and 
aided  by  movable  muscular  structures,  which  are 
known  as  lips.  These  aid  in  the  taking  in  of  food, 
or  in  retaining  it  when  it  has  entered  the  mouth 
cavity ;  in  the  production  of  sounds,  and  especially  of 
human  speech ;  and  they  are  an  important  factor  in 
the  production  of  the  expression  of  the  emotions. 

In  the  vertebrate  series  we  apply  the  term  teeth 
to  those  hard  bodies  which  are  set  in  the  mouth,  and 
are  developed  from  cells  of  epiblastic  origin  ;  in  their 
simplest  condition  these  organs  are  more  or  less 
simple  spiny  bodies,  exactly  comparable  to  the  spines 
which  are  found  on  the  skin  of  many  sharks.  Nor 
is  the  community  a  community  of  structure 
merely;  from  within  the  limits  of  the  history  of 
an  individual  it  is  possible  to  draw  sufficient  evidence 
to  prove  that  there  is  a  community  of  origin 
between  what  have  been  well  called  dermal 
denticles  and  what  we  call  teeth.  The  accom- 
panying figure,  which  represents  a  section  of  the  lower 


Chap.  IV.] 


TEETH  OF  VERTEBRATES. 


141 


jaw  of  a  dogfish,  at  a  stage  previous  to  that  at  which  any 
lip  is  developed,  shows  the  direct  continuity  of  struc- 
tures, which,  in  the  adult,  seem  to  be  very  different 
from  each  other.  When  we  consider  the  different  re- 
lations to  the  surrounding  parts  which  would  be  entered 
into  by  the  spines  that 
came  to  lie  within 
the  area  of  the  mouth, 
we  see  at  once  that, 
by  being  brought  into 
contact  with  the  food 
the  spines  would  be 
led  to  increase  in  size 
and  strength  \  but  this 
increase  in  activity 
would  be  necessarily 
accompanied  by  a 
richer  nervous  and 
vascular  supply;  and 
this,  reacting  on  the 
spines,  would  lead  to 
greater  differentia- 
tion, which  has  taken  the  form  of  greater  definiteness 
in  arrangement  and  structure. 

In  commencing,  therefore,  a  review  of  the  teeth 
of  vertebrates,  we  find  that  we  start  with  a  general- 
ised or  non-differentiated  condition  ;  as  we  pass  on 
we  shall  find  that  the  teeth  become  more  and  more 
limited  to  certain  bones,  and  diminish  in  number  ;  in 
other  words,  there  is  a  gradual  reduction.  Concur- 
rently with  this,  we  have  to  note  that,  when  a  group 
of  animals  becomes  especially  adapted  to  a  certain 
mode  of  life,  or  presents  marked  aberrations  from  the 
general  plan  of  structure,  they  become  edentulous, 
or  lose  their  teeth ;  such,  for  example,  are  the  pipe- 
fishes among  Fishes,  toads  among  Amphibia,  Chelonia 
(turtles  and  tortoises)  among  Reptiles,  all  recent 


62.  —  Section  of  Lower  Jaw  of 
young  Dog-fish,  showing  the  spines 
of  the  skin  under  the  jaw,  and  the 
teeth  ahove.  (After  C.  S.  Tomes.) 


142   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

birds,  the  duckbill  and  the  echidna,  and  some  of 
the  whales  among  Mammals;  this  is  a  phenomenon 
not  confined  to  Vertebrata,  for  it  may  be  observed  in 
the  Spatangoids  among  Echinoidea,  where  the 
"  Lantern  of  Aristotle  "  is  altogether  absent,  and  in 
the  tubicolous  Chsetopods,  which  have  lost  the 
strong  jaws  of  their  free-swimming  allies. 

In  correspondence  with  the  great  diversity  of 
mode  of  life  and  of  details  of  structure  among  fishes, 
we  note  in  that  group  the  very  greatest  differences 
in  the  disposition  and  size  of  the  teeth;  seeing,  indeed, 
here  an  excellent  illustration  of  the  law  that  com- 
mencing structures  are  subject  to  great  variability. 
Here,  too,  we  find  an  example  of  spines  on  the  skin 
taking  on  the  function  of  teeth  ;  the  true  teeth,  that 
is  to  say,  the  hard  structures  within  the  area  of  the 
mouth,  are,  in  the  saw-fish  (Pristis),  quite  small  and 
blunt ;  the  sides  of  the  enormous  snout  are,  however, 
provided  with  large  dermal  spines,  set  at  regular 
distances  from  one  another,  and  each  implanted  in  a 
special  socket. 

When  well  developed,  as  in  the  dog-fish,  the  teeth 
are  set  in  several  concentric  rows ;  those  of  the  outer 
are  alone  functional,  and  they,  as  all,  are  not 
attached  to  the  jaw,  but  are  only  fixed  in 
the  covering  membrane ;  this  membrane  appears 
to  move  over  the  surface  of  the  jaw,  and 
thereby  the  teeth  which  have  been  in  use  for  a  time 
are  removed  from  the  edge  of  the  jaw,  and  the 
next  succeeding  series  come  to  occupy  their  position, 
and  to  take  on  their  function. 

A  large  number  of  small  teeth  are  likewise  to  be 
found  in  many  bony  fishes  (Teleostei),  and  here,  where 
a  number  of  distinct  bones  are  developed^  we  often 
find  every  bone  within  the  mouth  bearing  teeth ;  as 
may  readily  be  supposed,  such  teeth  are  generally 
of  small  sizej  and  without  any  special  masticatory 


Chap.  IV.] 


TEETH  OF  FISHES. 


143 


a 


function  ;   indeed,    in    very  many  fishes  the  food  is 
swallowed  whole. 

Owing  to  the  fact  that  these  Vertebrates  are  not 
able  to  put  their  fins  to  the  duty  of  seizing  their  food 
in  the  way  in  which  many  higher  forms  use  their 
anterior  pair  of  limbs,  the  teeth  may  often  be  observed 
to  have  a  special  prehensile  function.  This  power  is 
sometimes  developed  to  an  extraordinary  degree ;  all 
the  numerous  teeth  in  the  mouth  of  the  pike  are 
directed  backwards,  and  so  prevent  or  oppose  the 
escape  of  any  prey  which  has  been  taken  into  the 
mouth  ;  an  extension  of  this  arrangement  has  been  de- 
scribed in  the  angler  (Lophius),  where  some  of  the 
larger  teeth  in  the  front  of  the  mouth  are  so  attached 
to  the  edge  of  the  jaw  that  they  spring  up  again  as 
soon  as  the  food 
which  has  pressed 
them  downwards 
into  the  mouth 
has  passed  them 
and  entered  the 
oral  cavi  ty 
(Tomes).  By 
this  means  the 
prey  is  caught  as 
in  a  trap. 

Where  the 
teeth  are  used  for 
the  purposes  of 
breaking  up  the  food  or  the  shell  in  which  it  is  con- 
tained, they  become  of  considerable  size,  as  in  the 
Wrasses  ;  in  the  parrot- Wrasses  (Scarus)  the  teeth  un- 
dergo fusion  with  their  neighbours ;  in  the  sheep's-head 
(Sargus)  the  teeth  in  the  front  of  the  mouth  are  cutting 
organs,  and  those  at  the  sides  larger,  and  have  their 
surface  rounded,  so  that,  as  they  move  on  one  another, 
they  act  as  grinders.  Where  a  number  of  teeth  are 


'ig.  63.— Lower  pharyngeal  Bone  of  Scants, 
showing  Teeth  of  different  Ages.  Two-thirds 
natural  size.  (After  C.  S.  Tomes.) 


144  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

required  they  are  not  always  confined  to  the  bones 
of  the  skull ;  thus,  in  the  just-mentioned  Scarus,  the 
lower  pharyngeal  bones  unite,  and  they,  like  the 
upper  pharyngeals,  are  armed  with  crushing  teeth  (Fig. 
63) ;  here,  then,  we  have  an  instance  of  the  bones  of  the 
branchial  arches  (see  page  328),  being  tooth-bearing. 
Another  example  is  afforded  by  the  carp,  in  which  fish 
the  bones  of  the  skull  are  all  devoid  of  teeth,  which 
are  confined  to  the  lower  pharyngeals ;  these,  as  in 
the  case  of  the  incisors  of  the  sheep  or  ox,  do  not 
work  on  upper  teeth,  but  on  a  hard  process,  which, 
in  the  carp,  is  developed  on  the  occipital  bone  of  the 
skull. 

In  other  fishes  the  teeth  are  exceedingly  deli- 
cate, as  in  Chcetodon,  which  has  gained  its  name 
from  the  bristle-like  character  of  these  organs.  In 
a  few  cases  the  teeth  are  placed  in  distinct  sockets, 
as  in  the  file-fishes,  of  which  Balistes  is  an  example ; 
in  Lepidosteus  the  socket  is  not  complete,  and  the 
tooth  becomes  anchylosed  to  its  walls. 

Lepidosiren  presents  an  arrangement  not  unlike 
that  which  is  found  in  Rodents  among  Mammals,  for 
the  front  edge  of  the  teeth  is  harder  than  the  rest, 
which  therefore  wears  down  sooner,  and  leaves  a  sharp 
cutting  edge.  In  no  group  of  the  Vertebrata  are  these 
organs  of  greater  value  to  the  palaeontologist  than 
among  Fishes,  as  the  discovery  in  Australian  rivers 
of  Ceratodus,  which  had  been  thought  to  have  been 
extinct  since  the  time  of  the  deposit  of  the  older 
secondary  rocks,  is  sufficient  to  bear  witness. 

In  a  few  cases  there  are  differences  between  the 
teeth  of  males  and  females,  as  in  the  skate  (Fig. 
64 ;  A  and  B)  where  those  of  the  male  are  more 
pointed  than  those  of  the  female. 

In  the  male  salmon,  at  the  breeding  season,  the 
lower  jaw  is  produced  into  stout  hooks,  and  in  corre- 
spondence with  this  the  anterior  end  of  the  upper  jaw 


Chap.  IV.J 


TEETH  OF  AMPHIBIA. 


is  also  enlarged,  and  the  premaxillary  teeth  are  four 
times  as  large  as  those  of  the  corresponding  region  in 
the  female. 

Small  and  simple  as  are  the  teeth  of  most  recent 
Amphibians,  they  are,  as  compared  with  those  of 
most  fishes,  greatly  reduced  in  number  ;  this,  no  doubt, 
is  largely  to  be  explained  by  the  development  of  the 


Fig.  64.— Teeth  of  Skate.    A,  Male  ;  B,  female. 


fore-limbs  into  organs  which  are  capable  of  seizing  and 
holding  the  prey,  or  of  pushing  it  into  the  mouth  ;  we 
find,  too,  that  the  great  majority  of  the  teeth  are  now 
found  on  the  membrane  bones  (see  page  329),  at  the 
sides  of  the  mouth  only;  though,  indeed,  the  frog  has  fine 
vomeriiie  teeth,  and  other  amphibians  have  them  on 
the  palatine,  or  the  pterygoid  bones. 

Whether  we  pass  now  to  the  Reptiles  or  to  the 
Mammalia,  we  get  still  more  marked  indications  of 
this  reduction  ;  in  all  the  latter  which  have  teeth,  and 
in  all  crocodiles  and  many  lizards,  teeth  are  found  only 
on  the  lower  jaw,  and  on  the  maxillse  and  premaxillse. 
K— 16 


146    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

As  may  be  supposed,  the  teeth  of  the  crocodile  are  of 
great  size  and  strength. 

An  instructive  example  of  a  quasi- edentulous  con- 
dition is  found  in  the  lizard  of  New  Zealand,  which  is 
known  as  Hatteria  ;  the  teeth  at  the  sides  of  the  jaw 
are  not  replaced  when  they  are  worn  down;  but  the 
bone  itself,  which  is  exceedingly  dense,  takes  on  the 
function  of  a  cutting  organ.  In  forms  which  are  perma- 
nently edentulous,  like  the  tortoise  or  the  pigeon,  the 
edges  of  the  jaws  become  invested  in  horn,  the  shape 
and  form  of  which  varies  with  the  habits  of  the 
animal ;  in  some  birds  (wild  duck)  the  edges  of  the 
horny  case  become  serrated  and  give  rise  to  the 
appearance  of  tooth- like  structures ;  in  some  cases 
(Odontopteryx)  the  edge  of  the  underlying  bone  be- 
comes denticulated  ;  these  are  adaptations  to  modes  of 
life,  and  must  be  carefully  distinguished  from  the 
actual  possession  of  true  teeth  such  as  characterises  a 
large  group  of  extinct  birds  (Odontomitlies). 

Curiously  resembling  the  arrangement  of  the  turtle, 
and  having,  of  course,  much  the  same  function,  are 
the  tough  horny  plates  on  the  jaws  of  tadpoles  ;  the 
history  of  these  plates  would,  however,  seem  to  be  very 
different  from  that  of  the  similarly  disposed  parts  in 
the  higher  forms ;  that  is  to  say,  the  beak  of  the 
tadpole,  and,  doubtless,  the  horny  apparatus  of  the 
lamprey,  aie  structures  which  preceded,  and  not  suc- 
ceeded, the  possession  of  teeth. 

A  phenomenon  similar  to  that  seen  in  Lophius  is 
to  be  observed  in  Snakes ;  here,  again,  the  organs  of 
prehension  being  absent,  owing  to  the  disappearance 
of  the  limbs  (see  page  96),  the  teeth  are  directed  back- 
wards ;  when,  therefore,  living  prey,  such  as  a  frog,  has 
entered  into  the  cavity  of  the  mouth,  it  is  prevented 
from  escaping  out  of  it  by  the  erection  of  the  teeth. 
Some  snakes  kill  their  food  by  constriction,  and  swal- 
low it  at  leisure  ;  others  swallow  it  whole,  and  in  them 


Chap.  IV.] 


TEETH  OF  REPTILES. 


the  bones  of  the  skull  are  loosely  connected  with  one 
another,  and  so  allow  of  the  enlargement  of  the  cavity 
of  the  mouth  ;  others,  finally,  kill  their  prey  by  biting 
and  simultaneously  injecting  poison  into  the  wound. 
In  these  last  (the  venomous  snakes)  there  may  be 
several  not  very  long  teeth  in  the  maxillary  bone,  or 
there  may  be 
but  one  maxil- 
lary tooth,  which 
in  such  cases  is 
long  and  mov- 
able. In  the 
former  the  fangs 
are  distinctly 
grooved  along 
some  part  of 
their  length,  but 
the  sides  of  this 

groove  are  suffi-  •"••-""Hr-WMte  /, 
ciently  close  to 
form  a  service- 
able canal,  along 
which  the  poison 
from  the  poison 
gland  may  make 
its  way  into  the 

WOlllld        In  the   ^^*  ^* — Transverse  Section  of  the  Poison  Farg 

latter  the  single  ],  Tooth  in  use;  2,  the  tooth  which  will  succeed  it;  3  to 
IQTTCO  ma  villar-tr  10,  tooth-sacs  numbered  in  the  order  of  their  succcs- 

large    maxillary       sion-  (After  c.  s.  Tomes.) 
tooth    appears, 

from  the  outside,  to  be  solid  and  ungrooved  ;  the  real 
fact  is,  however,  that  the  two  edges  of  the  groove  have 
completely  united  to  form  a  closed  canal,  the  existence 
of  which  becomes  apparent  in  a  transverse  section  of 
the  tooth  ;  the  opening  of  the  canal  is  not  placed  quite 
at  the  tip  of  the  sharp  fang,  but,  just  as  in  a  subcu- 
taneous injection  syringe,  the  orifice  is  a  little  behind 


148    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  tip  which,  as  it  were,  guides  and  saves  the 
poisonous  fluid. 

As  a  further  aid  to  the  more  extreme  venomous 
forms,  which  can  only  subsist  by  this  mode  of  attack- 
ing their  prey,  and  which  are  eminently  liable  to  have 
their  organ  of  offence  broken  in  the  act  of  "striking," 
the  reserve  teeth  are  arranged  in  a  manner  which 
seems  to  be  unique  in  the  animal  kingdom.  Instead 
of  a  single  series  of  reserve  teeth  set  in  one  and  the 
same  line  with  the  tooth  in  active  function,  there  are 
two  rows,  in  each  of  which  the  pair  of  teeth  are 
almost  of  the  same  age  and  grade  of  development. 
When,  therefore,  the  active  tooth  is  lost,  that  in  the 
other  line,  which  is  lying  beside  it  (Fig.  65),  is  ready 
at  once  to  move  forwards  into  a  little  different 
position,  and  to  take  on  its  function  ;  by  this  means 
the  fang  is  replaced  with  a  minimum  loss  of  time  (C. 
S.  Tomes). 

When  we  come  to  the  Mammalia,  where,  as 
has  been  already  said,  teeth  are  never  found  except  in 
the  mandibles,  maxillae,  and  premaxillae,  we  are  met, 
at  the  outset,  with  an  arrangement  of  which,  at 
present,  it  seems  impossible  to  afford  any  altogether 
satisfactory  explanation.  There  are  never  more  than 
two  sets  of  teeth,  one  of  which  is  temporary  or 
milk,  and  the  other  permanent*  the  teeth  of 
these  two  sets  differ  in  form,  size,  and  number. 
In  some  cases  the  milk  teeth  are  never  of  any  use  ; 
such  mammals  may  be  conveniently  spoken  of  as 
monophyodont,  while  those  in  which  there  are  two 
sets  may  be  similarly  called  diphyodont.  At  the 
same  time  it  must  be  carefully  borne  in  mind  that 
that  there  is  no  sharp  delimitation  between  these  two 
groups.  In  marsupials  and  guinea-pigs  there  is  only 
one  milk  molar ;  in  the  rabbit  the  milk  incisors  dis- 
appear before  birth,  and  among  edentates  only  one 
species  (Tatusia  peba)  is  known  to  have  milk-teeth. 


Chap,  iv.]  TEETH  OF  MAMMALS.  149 

The  definite  diphyodont  arrangement  is  Lest  seen  in 
the  higher  Mammals. 

The  possession  of  this  double  series  is  not,  how- 
ever, the  only  remarkable  character  of  the  teeth  of 
Mammals  ;  while  there  are  only  inconsiderable,  if  any, 
differences  in  the  form  of  the  teeth  of  any  given  fish 
or  reptile,  and  such  differences  are  characteristic  only 
of  small  groups,  we  find  that  for  a  large  number  of 
Mammals,  though  by  no  means  in  all,  the  teeth  in 
different  regions  of  the  mouth  have  distinctly  and 
definitely  different  forms  and  function;  (1)  in  the 
anterior  portion  we  find  sharp  cutting  teeth  ;  (2)  at 
the  sides  we  sometimes  see  strong  seizing  or  holding 
or  offensive  organs,  and,  farther  back  (3)  we  see  that 
the  upper  surface  of  the  tooth  becomes  widened  out 
and  tubercnlated  so  as  to  form  a  more  or  less  suitable 
grinding  surface.  Looked  at  in  a  general  way,  these 
three  kinds  or  forms  of  teeth  may  be  grouped  as  (1) 
incisors,  (2)  canines,  or  (3)  molars.  The  molars 
are  spoken  of  in  diphyodonts  as  premolars  or 
molars,  according  as  they  are  or  are  not  preceded  by 
milk  or  deciduous  molars.  Mammals  with  variously 
formed  teeth  are  conveniently  known  as  hetero- 
donts ;  while  a  nomodont  dentition  is  ascribed  to 
such  forms  as  the  edentates,  or  the  toothed  whales,  in 
which  all  the  teeth  have  exactly  the  same  character. 

When  a  homodont  dentition  obtains,  the  number 
of  teeth  in  the  jaws  may  be  very  great,  some  dolphins 
having  as  many  as  two  hundred  (Pontoporia)  ;  in  the 
other  forms  the  number  of  teeth  is  strictly  limited,  no 
known  living  mammal  having  more  than  forty-eight 
teeth  (Megalotis). 

In  comparing  the  teeth  of  one  heterodont  with 
those  of  another,  it  is  very  convenient  to  make  use  of 
the  set  of  symbols  which  make  up  the  "dental  for- 
mula ; "  here  the  letters  i,  c,  pm,  and  m,  represent 
the  different  categories  of  teeth,  while  the  fraction 


150    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

sign  is  used  to  represent  the  disposition  in  the  upper 
and  lower  jaws.  Making  use  of  this  method  of 
formulation,  we  may  represent  the  typical  dentition 
of  a  heterodont  mammal  thus  : 

.  3.3         1.1  4.4          3.3 

t   —  ,    c    —  ,    nm  —  ,    7»   —  =  44. 
3.3          1.1'    "      4.4'          3.3 

This  is  the  dental  formula  of  the  low  insectivorous 
mammal  Gymnura. 

The  dental  formula  of  man  is  : 

2.2        1.1  2.2          3.3 


And  that  of  the  cat  : 

3.3        1.1  3.3          1.1 

«  s73   c  TTi   pm  272    m  i7i  =  30< 

There  are  a  number  of  certain  modifications  in 
the  form  or  structure  of  the  tooth  which  at  once 
attract  attention.  While  those  Mammals,  such  as  man, 
which  bite  their  food  have  sharp  incisors,  those  that 
gnaw  it  have  the  greater  part  of  the  surface  of  the 
incisors  devoid  of  that  hardest  part  of  the  tooth 
which  is  called  the  enamel,  and  so  maintain  an  edge, 
the  softer  dentine  always  wearing  down  faster  than 
the  enamel  which  is  at  the  front  and  sides.  The 
teeth  which  are  set  at  the  outer  angles  of  the  jaw 
(the  canines)  are  especially  large  in  those  forms  which 
seize  on  a  living,  and  require  to  hold  a  struggling 
prey  \  and  it  is  these  organs  which  are  most  frequently 
converted  into  weapons  of  attack,  though  in  the  most 
prominent  case  of  all,  that  of  the  elephant,  the  tusks 
belong  to  the  incisor  series.  In  the  male  boars 
(Suidse),  the  canines  attain  to  a  considerable  length, 
being  in  the  Babirussa,  an  animal  not  so  large  as  the 


Chap,  iv.]  TEETH  OF  MAMMALS.  151 

domestic  pig,  from  eight  to  ten  inches  long ;  they  are 
often  exceedingly  sharp,,  and  are  capable  of  inflicting 
on  those  whom  they  attack  severe  and  deep  wounds. 
In  the  males  of  the  anthropoid  apes  the  canines  are 
always  much  larger  than  in  the  females,  and  as  they 
are  not  developed  till  later,  we  are  justified  in 
believing  that  they  are  sexual  weapons  of  attack,  by 
which  the  males  are  aided  in  fighting  with  one 
another  for  the  possession  of  the  females. 

The  molars  again  present  us  with  indications  of 
differences  in  the  form  of  the  teeth  corresponding  to 
differences  in  the  character  of  the  food ;  at  the  same 
time,  the  very  greatest  care  must  be  taken  in  esti- 
mating the  kind  of  food  from  the  form  of  the  teeth, 
and  at  all  times,  where  it  is  possible,  the  general 
arrangements  of  the  alimentary  canal  must  be  steadily 
borne  in  mind.  Four  chief  types  may,  however,  be 
easily  distinguished  (1)  carnivorous,  (2)  insecti- 
vorous, (3)  frugivorous,  (4)  herbivorous.  The 
molar  or  premolar  tooth  of  the  dog,  which  has,  since 
the  time  of  Cuvier,  been  distin- 
guished as  the  carnassial,  is 
modified  to  form  a  sharp  blade, 
and  is  continued  behind  into  a  thick 
tubercle  (Fig.  67;  A). 

The  typical  insectivorous  tooth 
is  distinguished  by  the  development 
of  four  or  five  sharp  cusps  (Fig.  66) ;  F3£  gj  Tootl1 
in  the  frugivorous  forms  the  cusps 
are  not  so  distinct,  nor  so  sharp,  and  are  more  con- 
nected with  one  another  by  more  or  less  distinct 
ridges. 

The  most  essential  point  in  the  arrangement  of 
the  herbivorous  or  grinding  molar  tooth  is  the  dis- 
position of  the  several  constituent  tissues  of  which  it 
is  made  up  ;  the  large  squarish  solid  structure,  formed 
of  pillars  and  ridges,  is  composed  of  enamel  and 


152    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

dentine  ;*  as  lias  already  been  said,  these  two  tissue 
differ  greatly  in  hardness  ;  they  will,  therefore,  wes 
down  unequally  and  so  give  rise  to  a  roughened  su 
face  well  adapted  for  grinding.  In  addition  to  tl; 


Fig.  67.— Teeth  of  Wolf. 

i  1  to  i  3,  Incisors ;  c,  canine ;  p  1  to  »  4,  preraolars  ;  m  1  to  TO  3,  molars  ;  h,  he 
of  the  first  molar  or  lower  "  carnassial"  tooth. 

outer  investment  of  cement,  we  find  it  also  filling  u 
the  interspaces  (Fig.  68).  In  the  Garni vora  th 
lower  jaw  is  so  articulated  to  the  skull  as  to  lc 
able  to  work  from  below  upwards ;  in  the  herbivoroi; 
forms  the  jaw  works  from  side  to  side. 

Some  of  the   Cetacea  (dolphins,  whales)  are  dii 
tinguished   by   the  total    absence   of   teeth,  and  th 

*  For  an  account  of  the  minute  structure  of  the  teeth,  s< 
Klein's  "Elements  of  Histology,"  chap.  xxi. 


Chap.  IV.] 


TEETH  OF  CETACEA. 


153 


special  armature  of  the  mouth  by  "  whalebone  ;  " 

but  indications  are  presented  both  by  palaeontology 

and  embryology  which  are  sufficient  to  justify  us  in 

believing  that  this 

order   was    primi- 

tively  provided 

with  a  heterodont 

dentition.      While 

the  dolphin  has  a 

number    of    teeth 

in  both  jaws  ;  the 

sperm-whale    has 

well-developed 

teeth  in  the  lower 

jaw    only;     the 

bottle-nosed  whale 

has   never    more 

than    four    func- 

tional teeth,  and  in 

the  narwal  (Mono- 

don)  it  is  the  male 

only  that  has  the 

ordinarily    single 

tusk     well    de- 

veloped ;  this  may 

be  as  much  as  nine 

feet  long.     Among 

the    whale-bone 

whales    teeth    are 

never    more    than 

rudimentary,    but 

in  the  adult  their 

place   is   taken   by   plates    of    "  baleen,"    which   are 

set  nearly  at  right  angles  to  the  axis  of  the  mouth, 

and  have  their  free  ends  frayed  out  into  a  number 

of  stiff  hairs,  which  make  a  most  efficient  strainer; 

the   whale,    in   taking   into   its   enormous   mouth   a 


Lower  Jaw. 


154    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

quantity  of  sea-water,  simultaneously  takes  in  a  large 
number  of  the  smaller  marine  animals;  raising  its 
tongue  it  drives  out  the  water,  but  retains  behind  the 
filter  the  food  that  came  in  with  it  (Fig.  70). 

In  all  vertebrates,  with  the  exception  of  some 
fishes  and  a  few  amphibians  (e.g.  Pipa),  there  is 
developed  in  the  floor  of  the  mouth  a  tongue,  which 
owes  its  primitive  origin  to  the  mucous  membrane 
which  covers  the  branchial  arches.  It  is  never  of 


Fig.  69.— Last  Molars  of  (A)  African,  (B)  Indian  Elephant. 

large  size  in  fishes,  and  in  them  is  never  supplied  with 
muscular  tissue  as  it  is  in  the  higher  forms ;  in  some 
cases  it  is  provided  with  a  horny  sheath. 

In  many  forms,  as  in  some  Amphibians  (e.g.  the 
frog),  where  it  is  attached  by  its  anterior  end  to  the 
symphysis  of  the  lower  jaw  ;  in  Chameleons,  where  it 
is  knobbed  at  its  extremity ;  and  in  various  other 
lizards,  where  it  is  cleft  anteriorly,  as  it  is  also  in 
Ophidia,  it  is  capable  of  considerable  protrusion,  and 
may  be  used  as  a  prehensile  organ. 

Among  Birds  the  tongue  is  protruded  with 
great  rapidity  by  the  wood -pecker  (Picidse),  and 
by  the  humming-birds  (Trochilidse),  and  sun-birds 


Chap,  iv.]  BALEEN.  155 

(Nectarinidse).    In  the  wood-pecker  some  of  the  hyoid 


Fig.  70.— Figures  to  illustrate  Position  and  Structure  of  the  Baleen. 
(From  Murie,  after  various  authors.) 

A,  Hind  view  of  skull  of  right  whale ;  w,  whale-bone ;  ra,  mesial  ridge  of  palate  ; 
J,  lower  jaw-none*  ;  B,  arch  of  baleen  plates,  as  seen  in  cross  section  of  the 
mouth  ;  c,  vertical  section,  showing  the  intermediate  substance;  is,  Baleen 
plates ;  b,  frayed  out  at  their  free  edges ;  D,  transverse  section  of  whale-bone, 
magnified. 


156    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

bones,  or  bones  that  support  the  tongue,  are  of  con- 
siderable length,  form  a  curve  with  its  concavity 
forwards,  and  then,  trending  forwards,  lie  on  the 
iipper  surface  of  the  skull,  and  reach  as  far  as  the 
nasal  region.  Attached  to  these  bones,  and  attached 
also  to  the  anterior  end  of  either  mandible,  is  an 
extensor  muscle,  which  lies  on  the  inner  or  concave 
side  of  the  loop ;  on  its  contraction  the  loop  is  drawn 
up,  and  as  the  only  mobile  point  of  the  hyoid  bones  is 
that  at  which  their  anterior  extremities  are  inserted 
into  the  tongue,  it  is  clear  that  the  upward  movement 
of  the  bones  must  result  in  the  forward  movement  of 
the  tongue  ;  in  some  other  Picidse  the  hyoid  bones 
are  movable  in  their  sheath,  but  the  result  is  the  same. 
Essentially  similar  arrangements  are  to  be  seen  in  the 
humming-birds  and  sun-birds.  The  mechanical  prin- 
ciple, that  the  longer  the  hyoid  bones  the  greater  the 
force  and  extent  of  the  protrusion  of  the  tongue,  is 
supported  by  what  has  been  observed  in  Picus  as 
compared  with  Zosterops  (Gadow). 

The  tongue,  thus  protruded,  is  in  the  woodpecker 
invested  in  a  horny  sheath,  which  ends  in  a  slender 
point,  and  is  provided  on  either  side  with  backwardly 
directed  prickles,  which  serve  to  draw  from  the  holes 
in  the  wood  the  insects  on  which  this  bird  feeds  ; 
their  capture  is  aided  by  the  slimy  secretion  of  the 
salivary  glands,  which  are  compressed  on  the  con- 
traction of  the  extensor  muscle.  (Compare  the  account 
of  the  Great  Ant-eater,  page  160.) 

In  the  sun-birds  the  horny  part  of  the  tongue 
forms  two  tubes ;  in  the  honey-eaters  there  may  be  as 
many  as  eight,  and  the  inner  or  outer  margins 
become  frayed  out  into  fine  processes;  in  the 
humming-birds  there  are  tubes,  the  edges  of  which 
are  ordinarily  entire.  It  has  been  pointed  out  by 
Gadow,  that  while  the  sucking  in  of  honey  is  an  easy 
process  when  there  is  sufficient  fluid  to  fill  the  anterior 


Chap,  iv.]          TONGUE  OF  VERTEBRATES.  157 

opening  of  the  entire  tubes,  air  would,  when  there 
is  not,  rush  in  inste'ad,  and  that  the  advantage  of  the 
frayed  margins  lies  in  the  fact  that  the  honey  will 
ascend  to  the  tubular  portion  by  capillary  attraction. 
It  is  important  to  note  that  these  arrangements  are  of 
a  homoplastic  character,  inasmuch  as  the  humming- 
birds are  not  close  zoological  allies  of  the  sun-birds ; 
in  other  words,  we  have  here  again  an  example  of 
how  similar  structures  are  gained  by  forms  which  live 
under  similar  external  conditions. 

In  the  Parrots  the  end  of  the  fleshy  tongue  is 
dilated,  and,  from  its  prehensile  function,  has  been 
compared  to  a  human  finger;  on  its  lower  surface 
there  is  a  broad  nail-like  horny  plate,  which  is  free  at 
its  anterior  border ;  in  the  brush-tongued  parrots 
there  are  spinous  papillae  on  the  upper  surface,  which 
stand  upright,  or  project  forwards  when  the  tongue  is 
protruded. 

The  tongue  of  Mammals  is  well  provided  with 
muscular  tissue,  so  that  it  is  not  only  protrusible  (it 
is  of  great  length  in  insectivorous  forms  like  the 
ant-eater),  but  is  capable  of  a  licking  movement; 
and  in  some,  such  as  the  lion,  it  is  armed  with  strong 
horny  papillae  that  are  of  considerable  assistance  in 
removing  flesh  from  bones.  In  man,  in  addition  to 
the  kind  of  functions  just  enumerated,  it  is  of  impor- 
tance as  an  accessory  organ  of  articulate  language.  In 
some  mammals  a  tongue  like  structure  (the  sufolingna) 
is  developed  beneath  the  tongue;  in  the  lemurine 
Galago  (Fig.  71),  the  front  end  of  this  organ  is  provided 
with  five  stiff  denticles,  arranged  in  comb-like  fashion, 
and  having,  apparently,  the  function  of  keeping  clean 
the  incisor  teeth  (Flower).  In  the  dog,  and  some  other 
Carnivora,  the  protrusible  tongue  is  supported  by 
a  fibrous  and  muscular  body,  the  so-called  "lytta," 
or  worm  of  the  dog's  tongue. 

The  walls  of  the  cavity  of  the  mouth  are  provided 


158    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


with  glands,  which  are  outgrowths  of  the  walls  them- 
selves ;  of  these  the  most  important  and  best  known 
are  the  salivary  glands  par  excellence ;  but  these 
are  only  specially  modified  forms  of  the  simple 
tubular,  which,  with  the  compound  tubular  glands, 
are  alone  found  in  the  lower  Vertebrata,  and  which 
have  at  first  no  other  function  than  that  of  lubri- 
cating the  tongue,  the  mouth  cavity,  and  the  food ; 

and  they  are, 
therefore,  only 
feebly,  if  at  all, 
developed  in 
fishes. 

In  the  Am- 
phibia the  most 
important  is  the 
intermaxillary  or 
internasal  gland 
(Wiedersheim), 
an  idea  as  to  the 
function  of  which 

Fig.  71.-Side  View  and  Lo*er  Surface  of  the    is  to    be   gathered 
Tongue  of  a  Gaiago.  from  the  fact  that 

in  the  water- 
living  Axolotl  it  is  but  feebly  developed,  while  in 
the  metamorphosed  and  pulmonate  form  known 
as  Amblystoma  it  is  of  much  larger  size.  In  this 
group,  further,  where  the  tongue  is  so  often  used 
as  an  organ  of  prehension,  the  lingual  glands  are 
well  developed,  and  their  secretion  is  driven  on  to  the 
surface  of  the  tongue  with  every  contraction  of  that 
organ »  In  the  chamseleons,  which  likewise  catch 
their  prey  by  the  aid  of  their  tongue,  the  labial  and 
palatine  glands  are  well  developed  ;  the  median  of 
these  latter  may  be  regarded  as  the  homologue  of  the 
intermaxillary  of  the  Amphibia.  The  glands  that 
lie  in  the  floor  of  the  mouth  are  ordinarily  well 


Chap.  IV.] 


SALIVARY  GLANDS. 


developed  in  the  Lacertilia,  and  very  richly  so  in  the 

poisonous  Heloderma  of  Mexico  ;  in  correspondence 

with  the  position  of  these  glands  the 

reptile  is  said  to  turn  on   its  back 

when  striking  its  prey  (Fischer) ;  the 

secretion    of    these    sub  maxillary 

glands  has  certainly  poisonous  effects, 

}}ut  the  blood  of  a  guinea-pig  killed 

by  it  does,  unlike  the  blood  of  the 

victim  of   the    bite    of  a    colubrine 

snake,  but  like  that  of  the  victim  of 

a    viperine     snake,     coagulate    after 

death   (Fayrer).       In    the  venomous 

Ophidia  the   poison    is   supplied    by 

the  labial  gland,  which  lies  along  the 

edge  of  the  upper  jaw,   and  has  its 

duct    opening     into    the     maxillary 

tooth. 

The  salivary  glands  are  small  in 
river  tortoises,  and  absent  in  the 
marine  Chelonia,  and  in  the  alligator, 
where  the  glands  are  confined  to  the 
tongue. 

In  Birds,  again,  lingual  glands 
are  well  developed,  but,  as  may  be 
supposed,  the  labial  are  altogether 
absent  ;  in  the  wood-peckers,  where 
the  tongue  is  protruded  with  great 
rapidity  (see  page  156),  the  sub- 
linguals  are  enormously  developed, 
and  provide  a  sticky  secretion,  which 
acts  like  bird-lime. 

In  the  Mammalia,  three  pairs  of  large,  ordinarily 
acinous,  glands  predominate  over  the  smaller  and 
more  scattered  bticcal  glands  ;  these,  from  their  posi- 
tions, are  distinguished  as  the  parotid,  submaxil- 
lary,  and  subliiiguals.  From  our  knowledge 


seen  from  below, 
showing  the  large 
Sublingual  Glands 
(i,  i),  the  Hyoid 
Bones  (e),  and 
the  Base  of  the 
Tongue  (/).  (After 
Macgillivray.) 


160    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

of  the  influence  which  the  secretion  of  these  glands 
has  on  starchy  foods  in  ourselves,  it  is  often  thought 
that  therein  lies  their  prime  function.  Certain  con- 
siderations seem  to  show  that  this  is  not  a  correct 
view ;  in  the  Cetacea  and  other  aquatic  forms,  and  in 
the  blood-sucking  Desmodus,  the  glands  are  con- 
siderably reduced  in  size ;  the  dog  "  bolts "  his  food, 
or,  in  other  words,  does  not  subject  it  to  the  influence, 
which  is  not  exerted  in  a  moment,  of  the  salivary 
secretions ;  while  the  kangaroo,  which  dwells  in  the 
arid  plains  of  Australia,  has  some  of  the  glands  of 
great  size.  It  would  seem,  therefore,  that  the  prime 
function  of  these  structures  is  to  afford  a  supply  of 
water  for  the  solid  food,  and  a  further  physiological 
advantage  is  gained  by  the  fact  of  this  water  being 
supplied  at  or  near  to  the  temperature  of  the  mam- 
malian body. 

The  well-known  fact  in  human  physiology  that 
the  secretion  of  the  parotids  is  the  most  watery,  and 
that  of  the  submaxillary  and  sublinguals  more  viscid, 
is  paralleled  in  comparative  anatomy  by  the  large  size 
of  the  parotids  in  animals,  such  as  ruminants,  and 
other  herbivorous  forms,  which  masticate  dry  foods ; 
and  the  great  size  of  the  submaxillary  in  such 
animals  as  the  Echidna,  or  the  ant-eater,  which 
require  a  viscid  fluid  for  the  purpose  of  catching  their 
insect  prey.  In  the  last-mentioned  mammal  some  of 
the  fibres  of  the  stylo-hyoid  muscle  encircle  the  sub- 
maxillary ducts,  which  are  thereby  constricted  when 
the  muscle  contracts,  and  in  this  way  the  ejection  of 
the  fluid  is  assisted  (W.  A.  Forbes). 

With  the  exception  of  the  buccal  cavity,  the  greater 
part  of  the  enteric  tract  of  a  Vertebrate  is  developed 
from  the  archenteron,  the  proctodeal  portion  being,  as 
a  rule,  exceedingly  short.  The  general  tract  may  be 
conveniently  divided  into  the  oasophagus,  stomach, 
intestine,  and  rectal  portion ;  this  last,  in  all  except 


Chap,  iv.]      INTESTINE  OF  VERTEBRATES.  161 

some  Fishes  and  the  higher  Mammalia,  opens  into  an 
epiblastic  pit,  the  cloaca,  into  which  there  also  open 
the  ducts  of  the  renal  and  generative  glands.  (See  page 
263.)  Primitively,  as  in  Amphioxus,  this  tract  is  quite 
straight,  and  has  no  definite  outgrowths  along  any 
parts  of  its  course  ;  later  on  it  undergoes  considerable 
modifications ;  its  anterior  region  gives  rise  to  an 
outgrowth,  which  serves  at  first  as  an  air  bladder, 
and  later  on  becomes  converted  into  a  pair  of  lungs, 
which  serve  as  the  definite  respiratory  organs  of  all 
higher  vertebrates  (see  page  236) ;  the  stomach  ceases  to 
have  its  long  axis  parallel  to  the  long  axis  of  the  body, 
and  at  the  same  time  becomes  enlarged  and  more  or 
less  complicated  ;  the  intestine  buds  off  two  glands  of 
high  physiological  importance,  the  liver  (only  feebly, 
if  really,  represented  by  a  caecum  in  Amphioxus)  and 
the  pancreas ;  the  intestine  becomes  narrower  an- 
teriorly than  it  is  posteriorly,  so  that  one  may  distin- 
guish a  small  and  a  large  intestine ;  between 
these  a  blind  outgrowth,  or  caecum,  which  in  Birds 
is  often  double,  is  nearly  always  developed ;  while 
the  inner  face  of  the  walls  of  part  of  the  intestine  is 
thrown  into  folds,  whereby  the  extent  of  the  absorbing 
surface  of  this  region  is  very  greatly  increased. 

So  far  as  the  intricacy  of  the  tract  is  concerned, 
we  find  it  to  be  a  very  general  rule,  not  only  in  Ver- 
tebrates, but  in  all  animals,  that  carnivorous,  as 
compared  with  herbivorous,  forms  have  a  simpler 
enteric  tract ;  thus,  the  gar-pike  (Belone)  has  the 
intestine  short  and  straight,  and  the  herbivorous  tad- 
pole has  a  more  complexly  coiled  intestine  than  the 
insectivorous  frog ;  the  proportion  of  the  length  of 
the  intestine  in  the  cat  is  to  the  body  as  from  3  to 
5  times  to  1,  while  in  the  pig  the  proportion  is  as 
12  to  1 ;  so,  too,  the  milk-fed  calf  has  a  much  less 
complex  stomach  than  the  grass-eating  and  ruminating 
cow. 

L— 16 


162    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

In  Amphibians  and  Reptiles  the  oesophagus  is  ordi- 
narily wide,  and  but  few  adaptive  modifications  are 
to  be  noticed  in  it.  Oligodon,  an  egg-eating  snake, 
presents  a  remarkable  arrangement.  It  will  be  easily 
seen  that,  were  this  animal  to  break  in  its  mouth  the 
eggs  on  which  it  feeds,  the  greater  part  of  the  contents 
would  almost  certainly  escape.  We  find,  however, 
that  the  teeth  in  the  mouth  are  quite  rudimentary, 
and  that  through  the  upper  wall  of  the  gullet  there 
project  the  elongated  inferior  spines  of  several  of  the 
vertebrse  of  this  region ;  the  tips  of  these  spines  are 
coated  with  a  substance  of  great  hardness,  and  the 
eggs,  after  they  have  reached  a  position  in  which 
their  contents  can  be  safely  disposed  of  by  the  animal, 
are  broken  by  them. 

The  most  important  modification  of  the  ossophagus 
is  to  be  found  in  birds,  and  especially  those  that  are 
grain-eating ;  there  is  here  developed  a  considerable 
enlargement,  the  so-called  "crop,"  which  may  not 
(cassowary),  or  may  (pigeon),  be  provided  with  special 
glands  in  its  walls.  The  object  of  the  former  arrange- 
ment would  appear  to  be  merely  that  of  a  kind  of 
reserve-pouch,  for  it  is  found  in  fish-eating  forms  and 
in  birds  of  prey.  On  the  other  hand,  the  glandular 
crop  is  a  necessity  for  such  birds  as  live  on  food  which 
is  so  difficult  of  digestion  as  is  grain* 

Throughout  the  whole  of  the  series  we  meet  again 
and  again  with  examples  of  a  stomach  only  slightly, 
if  at  all,  distinguished  by  its  size  from  the  rest  of  the 
tract ;  and  this  is  not  always  due  to  the  carnivorous 
habits  of  the  animal,  but  rather,  as  we  must  suppose, 
to  heredity,  and  to  the  fact  that  some  other  part  of 
the  intestine  performs  the  more  important  part  in  the 
functions  of  digestion.  Ceratodus  is  a  striking 
example  of  this,  for  here  the  stomach  is  almost  in 
a  straight  line  with  the  general  axis  of  the  body, 
while  the  interior  of  the  small  intestine  is  elaborately 


Chap,  iv.]  GIZZARD  OF  BIRDS.  163 

developed.  Two  leading  types  of  stomach  have  been 
distinguished  in  bony  fishes  ;  the  so-called  siphonal,  in 
which  the  two  halves  of  the  stomach  are  bent  upon 
themselves,  as  in  the  salmon,  and  the  csecal  form,  in 
which  the  upper  or  cardiac  portion  gives  off  a  long 
blind  sac.  The  walls  of  the  stomach  are  richly  pro- 
vided with  muscle  in  the  mullets. 

The  simplicity  of  the  resophagus  is  found  to  extend 
to  the  stomach  in  most  Amphibians  and  Reptiles,  and 
the  chief  modification  which  we  observe  is  an  increas- 
ing tendency  for  the  stomach  to  be  set  at  right  angles 
to  the  long  axis  of  the  body.  In  the  crocodile  the 
walls  are  very  muscular,  and  call  to  mind  those  of 
some  birds.  In  the  latter  group,  the  members  of 
which  are,  it  will  be  remembered,  toothless,  the  so- 
called  stomach  is  divided  into  two  more  or  less 
distinct  portions.  The  anterior  of  these  (the 
proventriculus)  has,  as  a  rule,  walls  which  are 
richly  provided  with  glands  that  exert  a  chemical 
action  on  the  food,  while  the  posterior  portion,  or 
gizzard,  has  walls  of  considerable  thickness,  owing 
to  the  great  development  of  its  muscles  ;  it  seems  to 
have  only  a  mechanical  action  on  the  food.  Two 
chief  types  of  gizzard  may  be  distinguished,  the 
simpler,  which  is  found  in  such  birds  as  are  carni- 
vorous or  insectivorous  or  live  on  soft  fruits,  has  the 
walls  much  less  thick  than  in  the  other  type ;  the 
muscles  radiate  out  from  two  tendinous  centres, 
which  are  placed  one  on  either  side  of  the  stomach, 
and  the  muscular  fibres  pass  from  one  to  the  other. 
The  crocodile  also  has  such  centra  tendiiiea. 

The  complicated  form  of  gizzard,  or  gizzard  par 
excellence,  is  distinguished  from,  the  more  simple  by 
the  great  development  of  its  lateral  muscles,  and  by 
the  conversion  of  its  lining  wall  into  a  strong  horny 
layer,  which  is  particularly  thick,  and  may  even  form  a 
hard  pad  on  either  side  (Fig.  73).  The  tendinous  patches, 


164    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

and  the  fibres  that  they  give  off,  are  very  strong.  We 
have  already  noted  the  presence  of  small  pebbles  in 
the  gizzard  ;  the  duty  of  these  is  to  act  as  grinding 
stones.  On  the  contraction  of  the  muscles  the  cavity 


Fig.  73.— Horizontal  Sections  of  the  Gizzard  of  a  Goose,  in  contraction 
(A)  and  relaxation  (B).  (After  Garrod ;  Proceedings  of  the  Zoolo- 
gical Society,  1872,  page  527.) 

of  the  gizzard  is,  of  course,  diminished  in  extent,  and 
the  food  contained  in  it  crushed  against  the  stones. 
It  sometimes  happens  that  the  lower  end  of  one  pad 
and  the  upper  end  of  the  other  are  respectively  better 
developed  than  the  rest ;  when  this  happens  a  slight 
sliding  is  added  to  the  crushing  movement,  which 
must  have  considerable  influence  in.  breaking  hard 
grains. 


Chap.  IV.] 


STOMACH  OF  MAMMALS. 


165 


In  the  heron,  which  lives  especially  on  fish,  which 
it  swallows  whole,  the  stomach  is  of  great  size  and 
extent,  reaching  nearly  to  the  anus,  and  occupying 
the  greater  part  of  the  abdominal  cavity. 

In  the  Mammalia  the  stomach  is  often  set  more 
or  less  at  right  angles  to  the  longitudinal  axis  of  the 
body,  and  its  upper  is  shorter  than  its  lower  curva- 
ture, which  may, 
therefore,  be  distin- 
guished from  one 
another  as  the 
greater  and  the 
less ;  the  enlarge- 
ment of  the  cardiac 
side  of  the  stomach 
is  a  little  more 
marked  in  the  dog 
and  in  man  than  in 
the  insectivorous 
Gymnura,  and  very 
much  more  so  in  the 
rabbit.  As  the  trans- 
verse axis  increases 
in  length,  a  division 
into  two  parts  be- 
comes more  or  less  well  pronounced ;  this  is  best  seen 
in  the  interior,  and  is  very  well  marked  in  the  case  of 
the  horse  (Fig.  74),  where  the  white-coloured  cardiac 
portion,  with  its  thick  epithelium,  is  separated  by  a  ridge 
from  the  redder  pyloric  sac,  with  its  softer  epithelium 
and  its  contained  gastric  glands.  This  separation  of 
the  stomach  into  a  reservoir  and  a  digestive  portion  is 
carried  to  an  extreme  in  the  ruminating  Ungulates. 

In  the  peccary  the  stomach  may  be  divided  into 
cardiac  and  pyloric  regions,  but  the  esophagus  is 
further  remarkable  for  being  continued  on,  in  the 
form  of  a  groove,  into  the  pyloric  division ;  in  addition 


Fig.  74.— Inner  Face  of  the  "Wall  of  the 
Stomach  of  the  Horse. 

a.  Cardiac ;  &,  pyloric  sac  ;  c,  duodenal  dilata- 
tion.   (After  Chauveau.) 


Pig.  75.— Stomach  of  a  Eliminating  Animal. 

A.  Exterior;  B,  interior;  a,  oesophagus  ;  b,  rumen  or  paunch;  c,  reticulum  or 
lioney-comb  bag  ;  d,  psalterium  or  manyplies ;  e,  abomasum  or  rennet- 
stomach. 


chap,  iv.i         STOMACH  OF  RUMINANTS.  167 

to  this,  a  well-marked  ridge  divides  the  cardiac  part 
into  two  regions.  In  the  chevrotain  (Tragulus)  the 
outer  portion  of  the  cardiac  division  forms  a  large 
paimcti,  and  is  incompletely  separated  from  a  portion 
lying  just  below  the  oesophagus,  the  inner  surface  of 
which  is  raised  up  so  as  to  form  a  honeycomb-like 
arrangement  of  ridges  ;  from  this  the  pyloric  diges- 
tive portion  opens  by  a  narrow  tube-like  piece.  In  the 
horned  ruminants  (Fig.  75)  this  tube-like  piece  becomes 
more  distinctly  separated  from  the  rest  of  the  pyloric 
portion,  and  its  inner  surface  becomes  raised  up  into 
a  number  of  folds  of  mucous  membrane,  which  are 
closely  appressed  to  one  another,  and  permit  only  of 
the  passage  of  the  most  finely  comminuted  food.  All 
gradations  in  complexity  are  to  be  observed  in  the 
size  and  number  of  these  lamellae  of  the  psalteriiim. 

In  the  desert-dwelling  camels,  and  in  their  allies, 
the  llamas  of  South  America,  part  of  the  cardiac 
region  is  converted  into  a  number  of  pouches,  which 
are  provided  with  sphincter  muscles,  by  which  they 
can  be  shut  off  from  the  rest  of  the  cavity  ;  these 
pouches  make  up  the  so-called  water-bag1  of  these 
animals. 

In  the  blood-sucking  bat  (Desmodus,  Fig.  76), 
where  little  digestive  secretion  is  required,  on  account 
of  the  nature  of  the  food,  the  pyloric  portion  of  the 
stomach  is  very  short,  but  the  cardiac  is  converted 
into  a  wide  caecum,  the  length  of  which  may  be  double 
that  of  the  body  of  the  animal,  and  which,  no  doubt, 
serves  as  a  reservoir  for  the  blood  that  is  sucked  in  as 
food. 

The  region  beyond  the  stomach  is  known  as  the 
intestine;  it  is  characterised  by  having  at  its  com* 
mencement  the  orifices  of  the  bile  ducts  from  the  liver, 
and  it  is  often  sharply  divisible  into  a  narrower  small, 
and  a  wider  large  intestine.  The  capacity  of  the 
internal  surface  is  increased,  as  in  the  stomach,  by  the 


1 68    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY, 

development  of  longitudinal  and  transverse  folds  :  the 
most  remarkable  of  these  ingrowths  is  one  that  we 
cannot  refrain  from  associating  with  the  typhlosole  of 
the  earthworm  and  of  the  fresh-water  mussel ;  when 
best  developed,  as  in  the  elasmobranchs,  this  fold 


Fig.  76. — Stomach  of  Desmodus,  in  the  form  of  a  long  caeca!  process. 


forms  a  spiral  valve.  In  a  more  rudimentary  con- 
dition it  is  presented  as  an  incomplete  spire  in  the 
lampreys,  as  consisting  of  thin  whorls  in  the  chi- 
msera,  or  of  varying  degrees  of  complication  in  various 
ganoids,  and  as  sometimes  forming  a  closely  appressed 
series  of  folds  in  the  skate.  Among  the  Teleostei  it 
is  very  rarely  developed,  though  a  projection  of  this 
kind  is  certainly  to  be  seen  in  some  specimens  of 
Chirocentrus,  and  has  been  expressly  said  to  be  found 
in  Butirinus  (Stannius).  In  the  Teleostei  there  appear 
tubular  outgrowths  of  the  upper  end  of  the  small 
intestine,  which  may  be  supposed  to  take  the  place  of 
the  absent  spiral  valve  ;  these  pyloric  appendages* 


Chap,  iv.]       ENTERON  OF  VERTEBRATES. 


169 


the  number  of  which  varies  considerably  in  different 
forms,  have  been  sometimes  found  to  be  full  of  food 
(Spatularia  Wieders- 
heim),  and  are  cer- 
tainly developed  in 
inverse  proportion  to 
the  spiral  valve  itself. 
They  are  only  one  of 
the  many  means  by 
which  vertebrate  ani- 
mals increase  the  ab- 
sorbing surface  of  their 
intestine  without  en- 
croaching greatly  on 
the  space  occupied  by 
the  other  organs  of  the 
abdominal  cavity. 

In  some  Amphibia, 
as  in  Siren,  the  differ- 
ence in  calibre  be- 
tween the  small  and 
large  intestine  is  not 
very  well  marked ;  in 
others,  as  the  frog, 
there  is  a  considerable 
difference.  The  same 
kind  of  difference  is 
seen  among  the  Rep- 
tilia ;  for  the  Ophidia 
and  the  Amphisbcena 
have  the  greater  part 
of  the  tract  of  the  same 
width,  and  the  whole  intestine  is  less  coiled  than  in  forms 
with  a  shorter  body,  such  as  the  Chelonia  or  the  Croco- 
dilia.  In  Birds  the  first  duodenal  loop  of  the  intestine 
always  encloses  the  pancreas ;  the  intestine  is  propor- 
tionately long,  but  while  nesting-birds,  such  as  the 


Fig.  77. — Diagram  of  the  General  Ar- 
rangement of  the  Abdominal  Portion 
of  the  Intestine  of  Mammalia,  seen 
from  in  front ;  the  small  intestine  is 
greatly  abbreviated.  (After  Flower.) 

p,  Pylorus ;  d,  duodenum  ;  i,  ileum ; 
r,  rectum. 


170    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

sparrow,  in  which  the  yolk  is  almost  or  altogether 
used  up  before  the  bird  leaves  the  egg,  rapidly  acquire 
the  enteric  proportions  of  the  adult,  those  which  leave 
the  egg,  like  the  chick,  before  the  yolk  material  has 
been  all  used,  are  longer  in  acquiring  the  proportions 
of  intestine  which  obtain  in  adult  forms  (Gadow). 

In  the  Mammalia  the  small  intestine  is  artificially 
divided  into  a  duodenum,  a  jejunum,  and  an 
ileum  ;  the  large  intestine  is  of  greater  length  than 
in  other  Vertebrates,  and  the  terminal  portion  only  is 
straight  or  rectal  (Fig.  77). 

The  rectum  in.  many  Fishes  does  not  open  into  a 
cloacal  pit,  but  lies  in  front  of  the  u  rino-genital  oritice  ; 
in  all  other  Vertebrata,  except  the  Eutheria,  the  rectal 
and  renal  orifices  open  into  a  cloaca,  while  in  the 
higher  Mammals  the  anus  lies  behind  the  urinogenital 
opening.  In  the  Chelonia,  where  the  walls  of  the 
abdomen  exert  little  or  no  influence  on  the  move- 
ments of  the  intestine,  part  of  the  terminal  portion  of 
the  gut  may  be  enlarged,  and  its  walls  provided  with 
abundant  muscular  tissue  (elephant  tortoise).  The 
walls  of  the  rectum  are  sometimes  provided  with 
glandular  appendages,  such  as  the  foursa  fabricii  of 
Birds,  and  the  bursar  anales  of  Chelonia ;  the  func- 
tion of  these  organs  is  unknown. 

The  large  is  sometimes  separated  from  the  small 
intestine  by  a  blind  ingrowth  or  diverticulum,  the 
ceecum ;  this  is  first  seen  in  the  Reptilia,  where  it 
varies  somewhat  in  size  in  different  forms,  but  it  is 
never  of  any  considerable  extent.  In  most  Birds  the 
caecum  is  double,  and  the  length  of  these  outgrowths 
are  in  direct  relation  with  that  of  the  intestine,  and 
therefore  with  the  habits  of  the  bird  ;  they  are,  in 
other  words,  longer  in  herbivorous  than  in  other  forms  ; 
it  has,  however,  been  pointed  out  by  Gadow,  that 
when  the  intestine  is  narrow  the  tract  is  long  and 
the  caeca  short,  while  other  short  caeca  may  also  be 


Chap,  iv.]  C&CA  OF  MAMMALS.  171 

associated  with  a  short  and  wide  intestine.  With 
these  caeca  there  must  not  be  confounded  the  frequently 
present  outgrowth  on  the  small  intestine,  which  is  the 
curiously  permanent  remnant  of  the  vitelline  duct, 
by  which  food  is  obtained  from  the  yolk. 

The  caecum  of  Mammals  presents  some  very  re- 
markable variations ;  and  we  find  many  exceptions  to 
the  generalisation,  that  it  is  short  in  carnivorous  and 
long  in  herbivorous  forms.  While  its  shortness  or 
absence  in  the  true  Carnivora  would  seem  to  afford  a 
support  to  the  generalisation  just  indicated,  it  is  more 
probable  that  its  small  size  or  absence  in  insectivorous 
forms,  is  rather  to  be  associated  with  the  dangers  to 
which  the  hard  chitinous  coverings  of  their  food 
might  expose  animals  that  fed  on  insects ;  the  same 
explanation  will  apply  to  the  insectivorous  bats,  and 
one  of  the  same  sort  to  the  frugivorous  species  of 
Chiroptera,  or  to  the  fish-eating  otter.  At  the  same 
time,  it  is  not  to  be  thought  that  all  frugivorous 
mammals  are  without  caeca ;  not  only  has  man  a 
caecum,  but  the  fruit-eating  monkeys  have  one  also. 
Herein  we  find  one  of  the  great  arguments  against 
prophesying  as  to  the  habits  of  an  animal  from  what 
we  can  see  as  to  its  structure,  and  find  a  sufficient 
answer  to  those  who  assume  that  the  parts  of  an 
animal  are  in  accordance  with  its  habits.  The  tele- 
ologist  takes  no  account  of  that  influence  which,  as  we 
have  shown  again  and  again,  is  no  less  an  important 
factor  than  adaptation,  the  factor  of  heredity. 

It  is  clear  enough  that  a  cherry-stone  impacted  in 
a  narrow  caecum  may  produce  inflammation,  and  yet 
an  animal  that  has  been  so  successful  in  the  struggle 
for  existence  as  has  man,  has  not  only  a  caecum 
(which  is  about  two  or  two  and  a  half  inches  in.  width), 
but  this  is  continued  into  an  appendix  vermiformis, 
which  is  not  so  much  as  half  an  inch  in  width.  So 
far  as  man  is  concerned,  we  may  well  suppose  that  he 


172    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

has  inherited  this  structure,  for  it  is  found  also  in  all 
the  anthropoid  apes,  while  its  wider  distribution 
among  the  ancestors  of  the  present  Mammalia  is 
spoken  to  by  the  presence  of  a  vermiform  prolongation 
to  the  caecum  in  the  rabbit.  An  appendix  which  is 
physiologically,  even  if  it  be  not  morphologically  com- 
parable to  that  of  a  man,  is  to  be  seen  in  the  wombat 
among  the  Marsupialia. 

Though  it  is  not  correct  to  say  that  the  caecum  is 
largest  in  herbivorous  mammals,  for  it  is  absent  in  the 
bears,  who  are  the  most  herbivorous  of  the  Carnivora, 
yet  in  many  cases  there  is  a  certain  relation  between 
the  size  of  the  caecum  and  that  of  the  stomach ;  for 
example,  in  the  rabbit  the  stomach  is  simple,  while 
the  caecum  is  enormous  (more  than  18  inches  in  length), 
and  its  absorbent  surface  is  increased  by  the  develop- 
ment in  its  interior  of  a  large  spiral  valve  ;  similarly, 
the  horse  has  a  very  large  caecum  ;  on  the  other  hand, 
the  Ruminantia,  in  which  the  stomach  is  so  complexly 
arranged,  have  a  simple  caecum.  It  would,  therefore, 
appear  that  when  the  absorbing  surface  of  the  stomach 
is  not  greatly  increased  by  adaptive  modifications,  a 
special  region  of  the  intestine  is  entrusted  with  the 
functions  that  the  stomach  cannot  perform. 

In  Hyrax  there  are  two  caeca. 

In  all  Vertebrates  the  liver  appears  to  arise  as  a 
bud  or  outgrowth  from  the  wall  of  the  enteron,  and 
this  embryonic  condition  is  that  which  is  seen  in  the 
lancelet,  where,  probably,  the  organ  has  no  definite 
function ;  in  most  of  the  higher  forms  the  secretion 
from  the  cells  is  stored  up  in  a  receptacle  connected 
with  the  bile  ducts ;  this  is  known  as  the  gall 
bladder,  and  it  is  generally,  though  not  invariably, 
attached  to  the  right  lobe.  The  primitive  bud  veiy 
early  becomes  double,  though  this  is  effected  in  differ- 
ent ways ;  in  the  chick  the  two  diverticula  are  at  first 
unequal,  in  sharks  and  Amphibians  the  primary 


Chap.  IV.) 


LIVER  OF  MAMMALS. 


'73 


outgrowth  becomes  bilobed,  while  in  the  rabbit  two 
diverticula  are  developed,  but  not  simultaneously. 

The  fat  formed  in  the  liver  is,  in  many  Fishes,  fluid  ; 
or,  in  other  words,  oil ;  in  these  animals  the  organ  is 
often  of  large  size  in  proportion  to  the  rest  of  the 


iff 


Tig.  78.—  Diagramatic  View  of  the  Inferior  or  Visceral  Surface  of  a 
Multilobed  Liver  of  a  Mammal  extended  transversely. 

tr,  Inferior  vena  cava;  p,  vena  portae;  it,  umbilical  vein  of  the  foetus,  represented 
by  the  round  ligament  in  the  adult,  lying  in  the  umbilical  fissure ;  77.  left 
lateral  lobe  ;  Ic,  left  central  lobe :  re,  right  central  lobe ;  rl.  right  lateral  lobe ; 
s,  spigelian  lobe ;  <-,  caudate  lobe ;  g,  gall  bladder ;  dr,  remnant  of  ductus 
venosus  ;  llf,  left  lateral  fissure ;  c/,  central  fissure  ;  rlf,  right  lateral  fissure. 
(After  Flower.) 

intestines,  and  much  less  firm  in  its  consistency  than 
in  the  higher  divisions  of  the  Vertebrata. 

With  regard  to  the  liver  of  Mammals  (Fig.  78), 
various  attempts  have  been  made  to  form  a  satisfactory 
system  of  nomenclature  for  its  lobes,  but  it  remained 
for  Flower  to  suggest  one  which  should  gain  universal 
acceptance, 

We  are  taught  by  embryology  that  the  liver 
ordinarily  arises  by  two  lateral  outgrowths  from  the 
intestine,  so  that  it  is  primitively  bilobed ;  in  the 
young  (foetus)  the  umbilical  vein  divides  the  liver 


174    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

into  two  parts,  and  this  vein  is  retained  in  a  rudi- 
mentary condition  in  the  adult  as  the  round,  liga- 
ment. It  may,  therefore,  be  taken  as  lying  in  the 
middle  line  of  the  liver ;  the  parts  or  lobes  that  lie  on 
one  side  are  the  right  lobes,  those  that  lie  on  the 
other  are  the  left  lobes.  The  fissure  in  which  the 
remains  of  the  umbilical  vein  lie  is  called  the 
umbilical  fissure,  but  it  is  not  always  the  deepest ; 
where  a  lateral  fissure  is  as  deep,  or  deeper,  the 
liver  appears  to  lose  its  bilobate  character,  and  seems 
often  to  consist  of  three  chief  parts.  The  characters 
and  relations  of  the  umbilical  fissure  must,  therefore, 
be  carefully  borne  in  mind  if  we  desire  to  retain  the 
proper  morphological  conception  of  the  liver  as  an 
originally  bilobecl  organ.  A  well-marked  lateral 
fissure  being  frequently  found  on  either  side  of  the 
umbilical,  it  results  that  the  organ  is  often  found  to 
consist  of  four  chief  lobes ;  these,  from  their  topo- 
graphical relations,  may  be  spoken  of  as  right 
central,  left  central,  right  lateral,  and  left 
lateral ;  while  the  whole  mass  of  lobes  on  either 
side  of  the  primitive  middle  line  may  be  called  re- 
spectively the  right  and  left  segments. 

So  far,  then,  we  find  that  the  liver  is  an  organ 
consisting  of  two  segments,  each  of  which  may  be 
divided  into  two  or  more  lobes.  All  the  more  impor- 
tant modifications  of  the  liver  affect  its  right  segment; 
with  the  right  central  there  is  very  frequently  con- 
nected a  reservoir  or  gall  bladder ;  the  right  lateral 
often  develops  a  prolongation  on  its  lower  surface, 
which  is  known  as  the  spigelian  lobe ;  another 
accessory  to  be  developed  from  the  right  lateral  is  the 
so-called  caudate  lobe. 

While  all  these  parts  are  to  be  found  in  the  human 
liver,  we  find  some  considerable  variations  exhibited 
in  different  mammalia ;  in  some  cases  (Cetacea,  Peris- 
sodactyla,  and  some  other  Ungulata,  etc.)  the  gall 


chap,  iv.]          LIVER  OF   VERTEBRATES.  175 

bladder  is  absent,  though  the  ducts  may  be  enlarged 
at  their  extremity;  sometimes  (hippopotamus)  it  is 
present  or  absent,  and,  in  the  lemurs,  it  may  be  seen 
on  the  convex  aspect  of  the  liver.  Sometimes  the 
segments  of  the  liver  are  greatly  subdivided,  and  there 
are  considerable  differences  in  the  depth  of  the  fissures; 
thus,  in  the  porpoises,  the  two  segments  are  subequal 
and  no  further  divided,  while  in  the  seal  there  are  a 
number  of  minute  notches. 

The  liver  is  often  adapted  to  the  form  of  the  body 
of  its  possessor,  being  elongated  in  elongated,  truncated 
in  shorter  forms  ;  in  the  lower  Vertebrates  it  largely 
retains  its  primitive  bilobate  character.  The  ducts  by 
which  its  secretion  passes  into  the  intestine  vary  con- 
siderably in  number  and  arrangement,  and  even  closely 
allied  forms  may  or  may  not  be  provided  with  the 
reservoir  which  is  known  as  the  gall  bladder. 

In  Vertebrates  with  "  hot  blood  "  the  bulk  of  the 
liver  is,  in  proportion  to  the  size  of  the  animal,  less 
than  in  the  so-called  cold-blooded  orders ;  this,  no 
doubt,  is  to  be  associated  with  the  greater  demand 
which  is  made  by  the  former  on  the  store  of  fat  which 
accumulates  in  the  liver,  and  which  is  more  rapidly 
used  up  by  them  than  by  those  animals  in  which 
oxydation  is  less  extensive.  The  observations  that 
have  been  as  yet  made  on  the  "  glycogenic  "  function 
of  the  liver  have  been  directed  rather  to  a  study  of 
the  mode  of  production  of  this  starchy  compound 
than  to  the  differences  which  obtain  in  different  groups 
of  Vertebrates. 

A  special  outgrowth  of  the  wall  of  the  intestine 
gives  rise  in  most  Vertebrates  to  an  important  diges- 
tive or  ferment-producing  organ,  the  pancreas.  It 
is  ordinarily  connected  with  the  duodenal  region  of 
the  intestine,  into  which  its  duct  often  directly  opens  ; 
in  other  cases,  as  in  the  frog,  the  duct  opens  into  the 
bile  duct.  The  details  of  the  comparative  physiology 


176    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

of  this  organ  still  await  investigation,  but  sufficient  is 
now  known  as  to  its  importance  to  lead  us  to  hope  for 
much  instructive  information  as  to  the  action  of  its 
secretion  in  different  animals. 

Although  it  is  absolutely  certain  that  structural 
characters  are  profoundly  modified  by  changes  in 
function,  or,  in  the  words  of  John  Hunter,  "  I  dare 
say  the  different  manner  of  living  gives  rise  to  the 
different  formation  of  the  viscera,"  it  is,  on  the  other 
hand,  a  fact  beyond  contradiction  that  in  two  purely 
herbivorous  animals,  such  as,  for  example,  the  horse 
and  the  cow,  or  piscivorous  forms  as  the  seal  and  the 
porpoise,  we  find  anatomical  structures  which  are 
strikingly  different ;  to  understand  this  we  must  again 
invoke  the  principle  which,  as  we  have  already  said, 
stands  equal  in  value  to  that  of  the  power  of  varying 
with  varying  circumstances ;  certain  modifications  of 
structure  are  impossible  to  certain  animals,  on  account 
of  the  influence  of  heredity  ;  in  other  words,  descent 
as  much  as  environment  has  to  be  taken  into  account 
in  the  study  of  the  morphological  characters  of  the 
parts  of  any  organism. 

The  first  definite  evidence  as  to  the  influence  of 
food  on  the  structural  characters  of  the  digestive  canal 
was  given  by  John  Hunter,  when  he  fed  a  sea-gull  for 
a  year  on  barley,  and  found  that  the  muscular  tissue 
of  the  gizzard  became  enormously  developed. 

It  is  stated  that  "this  experiment  is  annually 
repeated  by  Nature;  that  the  herring-gull,  Larus 
tridactylus,  of  the  Shetland  Islands,  twice  every  year 
changes  the  structure  of  its  stomach  according  to  its 
food,  which  consists  during  the  summer  of  grain  and 
in  winter  of  fish."  Somewhat  similar  observations 
have  been  made  on  the  raven  and  the  owl,  and  the 
converse  experiment,  or  that  of  converting  the  stomach 
of  the  grain-eating  pigeon  to  the  carnivorous  type,  has 
been  effected  by  Holmgren. 


Chap    IV.] 


PARASITIC  HABITS. 


177 


The  influence  of  the  parasitic  mode  of  life  on 
the  organs  of  digestion  is  exceedingly  well  marked. 
One  of  the  most  obvious  and  common  results  is  the 
loss  by  endoparasites  of  a  mouth  ;  this,  among  the 
Protozoa,  obtains  in  the  Gregarines,  which,  living  in 
organs  or  cavities  of  other  animals  (such  as  the  intes- 
tine of  the  lobster,  or  the  testicular  reservoirs  of  the 
earthworm)  that  are  rich  in  nutrient  fluids,  obtain 
their  necessary  nourishment 
through  their  cuticle  by  the  merely 
physical  process  of  osmosis. 
Among  the  ciliated  .Infusori  an  s, 
Opalina  is  mouthless.  The  same 
phenomenon  is  seen  among  the 
Metazoa  in  Echinorhynchus  and 
the  Cestoda,  which  in  their  adult 
condition  live  always  in  the  di- 
gestive cavity  of  Vertebrates. 
What  is  certainly  true  of  Tsenia 
serrata  (Fredericq),  namely,  that 
no  digestive  ferment  is  to  be  found 
in  anv  part  of  its  body,  is  doubt-  Fig.  79.— Atineta  tube- 
less  true  also  of  other  Cestoda,  ^VaextendelZd 
and  is  to  be  explained  by  the  retracted, 
fact  that  these  animals  live  in  the 
midst  of  food  which  is  being  made  ready  to  pass 
through  animal  membranes. 

In  another  large  set  of  cases  food  is  obtained  by 
suction;  among  the  Protozoa  this  is  seen  in  the  ecto- 
parasitic  Acinetse  (Fig.  79),  where  elongated  tubular 
processes  of  protoplasm  arise  from  the  surface  of 
the  body ;  these  tentacles,  as  they  are  often  called, 
are  capable  of  very  rapid  protrusion ;  their  knobbed 
ends  widen  into  sucking  discs,  and  are  able  to  pene- 
trate the  cuticle  of  their  prey,  which  are  ordinarily 
ciliated  Infusoria  ;  the  semifluid  endosarc  is  then  drawn 
up  through  the  granular  axis  of  the  sucking  tube. 

M-16 


178  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Among  the  Metazoa  sucking  tubes  are  best  developed 
in  the  Rhizocephala;  these  parasitic  Crustaceans  begin 
life  in  the  shape  of  free-swimming  naupliiform  larvae 
(see  page  534),  attach  themselves  to  the  bodies  of 
higher  Crustacea,  and,  losing  all  appearance  of  having 
true  appendages,  develop  at  their  anterior  end  a 


Fig.  80. — Sacculina,  carcini.  A,  Adult  form  showing  the  tuft  of  roots 
which  it  insert*  into  the  boJy  of  its  host.  B,  The  naupliiform  larva. 
(After  Haeckel.) 

number  of  filamentar  processes.  These  make  their 
way  into  the  body  of  the  host,  and  by  endosmosis 
take  up  nutriment,  which  they  pass  on  to  the  shapeless 
body. 

The  mouth  and  the  commencement  of  the  diges- 
tive tract  are  often  adapted  for  sucking,  as  in  the 
liver-fluke,  where  the  contractions  of  the  protractor 
and  retractor  muscles  of  the  pharynx  effect  this  pur- 
pose ;  in  the  JVematoid  worms,  where,  as  in  the 
fluke,  the  intestine  is  complete,  and  is,  moreover, 
provided  with  an  anus,  muscles  set  radially  around 
the  oesophagus  extend  to  the  walls  of  the  body ; 


Chap,  iv.]  SUCTORIAL  HABITS.  179 

on  their  contraction  the  cavity  of  the  resophagus  is 
greatly  increased,  and  a  vacuum  is  thereby  formed; 
here,  then,  the  tube  acts  as  a  sucking-pump.  The 
great  care  needed  in  making  any  generalisations  in 
comparative  physiology  is  well  spoken  to  by  the 
curious  fact  that  among  the  parasitic  Nematoids  a 
mouthless  condition  obtains  only  in  the  free-living 
stages  ;  thus  Gordius  has  a  mouth  when  parasitic, 
but  when  it  leads  a  free  life  the  mouth  is  lost,  and  the 
worm  is  dependent  on  the  store  accumulated  in  its 
earlier  stage ;  in  its  ally,  Mermis,  the  peri-o3sophageal 
muscles  are  lost  in  the  free  stage  of  existence,  though 
the  mouth  remains.  In  the  leech  the  sucking  action 
is  effected  in  essentially  the  same  way  as  in  the 
Nematoid.  In  the  scorpion,  where  the  mouth  is  ex- 
cessively minute,  the  pharynx  is  pear-shaped,  and  has 
attached  to  its  wall  transversely  set  muscles,  which, 
on  contraction,  increase  the  extent  of  the  cavity,  and 
so  cause  a  vacuum  which  results  in  an  up-flow  of  the 
fluids  of  the  prey  which  it  has  stung  to  death.  A 
large  number  of  Araclmida  have  a  distinct  sucking 
apparatus. 

The  changes  induced  by  parasitic  habits  on  the 
conformation  of  external  parts  of  the  body  are,  as 
may  be  supposed,  most  striking  in  the  Artliropoda, 
Among  the  Entomostraca  a  very  instructive  series  of 
gradations  may  be  made  out.  The  gnathites  of 
Cyclops  are  in  Caligus,  which  is  a  temporary  parasite 
on  fishes,  enclosed  in  a  tube  formed  by  the  fore-and- 
hind-lips  ;  the  anterior  pair,  or  mandibles,  are  alone 
well  developed,  and  form  piercing  processes.  In 
Corycseus  the  suctorial  tube  is  not  developed.  In  both 
of  these  there  are  swimming  feet.  In  Lerneea,  which 
may  be  found  on  the  gills  of  the  cod,  to  which  the 
adult  females  are  permanently  attached,  the  swimming 
feet  are  small.  In  Achtheres,  which  is  found  on  the 
perch,  these  feet  are  wanting,  and  a  pair  of  gnathites 


180  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

unite  to  form  a  single  sucker,  or  disc  of  attachment. 
As  we  pass  from  Cyclops  to  Lernsea  or  Achtheres,  the 
form  of  the  body  becomes  less  and  less  obviously  seg- 
mented, and  more  and  more  bizarre  in  appearance.  In 
Argulus,  the  common  parasite  of  the  stickleback  and 
other  fishes,  the  changes  are  still  more  marked  ;  the 
suctorial  tube  is  large,  and  has  within  a  pair  of  finely 
toothed  mandibles  and  style -shaped  maxillae ;  in 
front  of  the  mouth  is  a  pointed  spinous  tube,  contain- 
ing the  ducts  of  what  are  supposed  to  be  poison 
glands  ;  the  anterior  pair  of  maxillipeds  (Glaus)  form 
large  sucking  discs  on  either  side  of  the  mouth. 

Among  the  higher  Crustacea  the  parasitic  forms 
belong  to  the  group  of  the  Isopoda ;  the  Bopyridse 
have  a  sucking  proboscis,  and  their  mandible  is  with- 
out a  palp ;  some,  like  Entoniscus  and  Cryptoniscus, 
are  ordinarily  lernseoid  in  form,  when  adult ;  one 
species  of  the  latter,  which  is  parasitic  on  a  Sacculina, 
which  is  parasitic  on  a  Pagurus,  has  so  peculiar  a 
form  as  to  have  received  the  specific  name  of  plana- 
roides. 

Among  the  Arachnida,  many  of  the  mites 
(Acarina)  are  parasitic,  and  the  bases  of  the  two  an- 
terior pairs  of  appendages  form  a  sucking  proboscis, 
as  in  Demodex,  which  dwells  in  the  hair  follicles  of 
various  mammals ;  the  females  of  itch-mites  are  able 
to  bore  under  the  skin  ;  in  the  blood-sucking  ticks  the 
proboscis  is  provided  with  a  number  of  hooks.  In 
Pentastomum,  which  has  two  hosts,  and  is  endo-para- 
sitic  in  both  of  them,  the  only  signs  of  appendages  to 
the  body  are  the  two  hooks  on  either  side  of  the 
mouth. 

Parasitism  is  very  rare  among  Molliisca ;  Mont- 
acuta  lives  among  the  spines  of  Spatangus,  Stylifer  on 
sea-urchins,  among  corals,  or  in  starfishes,  but  neither 
of  these  are  specially  modified.  Entoconcha,  parasitic 
in  Holothurians,  is  merely  known  as  an  ovigerous  sac. 


Chap,  v.]  THE  BLOOD.  181 

No  Vertebrate  is  truly  parasitic,  for  Myxine 
(the  "  Borer ")  penetrates  the  body  of  other  fish  for 
the  direct  purpose  of  feeding  on  their  flesh  ;  and  the 
relation  of  Fierasfers  to  the  Medusae,  Echinoderms,  and 
Molluscs,  with  which  they  have  been  found,  is  only 
that  of  a  guest  which  makes  use  of  the  currents  of 
water  which  the  host  intended  for  its  own  purposes 
(SYMBIOSIS). 


CHAPTER  V. 

THE    BLOOD    AND   THE   BLOOD-VASCULAR   SYSTEM. 

THE  result  of  the  process  of  digestion  is  the  formation 
of  a  quantity  of  material  which  can  be  usefully  taken 
up  by  the  different  cells  of  which  the  body  is  com- 
posed, and  used  by  them  for  the  purposes  of  repair, 
growth,  and  reproduction.  In  a  majority  of  cases  the 
material  has  yet  another  function,  inasmuch  as  it 
becomes  the  vehicle  for  the  oxygen  which  is  constantly 
necessary  to  cell-activity ;  it  is  respiratory  as  well 
as  nutrient. 

This  material  takes  the  form  of  a  liquid  in  which 
cells  or  corpuscles  float ;  and  these  cells  are  either 
colourless,  or  tinged  red  by  haemoglobin,  and 
are  either  amoeboid  or  constant  in  form.  The 
liquid  is  known  as  the  plasma,  and  it  is  either 
colourless,  or  tinged  red,  green,  or  blue. 

The  different  characteristics  of  the  various  parts  of 
this  nutrient  and  respiratory  medium  are  most  clearly 
seen  in  the  Yertebrata,  with  which,  therefore,  we  will 
commence. 

The  blood  is  a  fluid  containing  white  and  red  cor- 
puscles, a  certain  amount  of  dissolved  albuminous 
and  mineral  bodies,  and  about  half  its  own  volume  of 


1 82  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

gases  \  the  red  would  appear  to  be  derived  from  the 
white  corpuscles,  and  some,  at  any  rate,  of  these  last, 
owe  their  origin  to  the  colourless  amreboid  corpuscles 
of  the  lymph;  this  lymph,  again,  is  finally  depen- 
dent for  the  production  of  fresh  corpuscles  on  the 
thicker  milky  fluid  which  is  found  in  the  lymphatics 
of  the  intestines  (chyle) ;  and  this  chyle  is,  of  course, 
due  to  the  metamorphosis  of  the  food  taken  into  the 
digestive  tract,  and  there  converted  into  peptones  and 
other  diffusible  bodies. 

The  white  or  colourless  corpuscles  are  amreboid  in 
form,  vary  a  good  deal  in  size,  but  are  constantly 
smaller  in  Mammals  than  in  other  Vertebrates,  and 
while,  on  the  whole,  less  numerous  than  the  red  cor- 
puscles, they  vary  in  number  according  to  the  state  of 
lasting  or  repletion. 

In  all  Vertebrates  save  Mammals  the  red  blood 
corpuscles  are  provided  with  a  nucleus ;  in  all  Mam- 
mals, except  the  camel  and  the  llama,  the  red  discs  are 
circular  in  form,  as  they  are  also  in  the  cyclos  tomato  us 
fishes ;  in  the  remaining  Vertebrates  the  discs  are 
elliptical.  These  red  corpuscles  differ  very  greatly  in 
size  ;  largest  in  the  urodelous  amphibian  Amphiuma, 
where  they  measure  about  -^  millimetre  by  -^ ;  they 
are  smallest  in  the  Qhevrotain  (Tragulus),  where  they 
are  only  —^  millimetre  in  diameter.  The  interesting 
researches  of  Gulliver  have  shown  that,  within  the 
limits  of  any  given  natural  group,  the  corpuscles  are 
largest  in  the  largest,  and  smallest  in  the  smallest 
species  of  the  group.  The  average  number  of  red 
corpuscles  in  a  cubic  centimetre  of  human  blood  has 
been  estimated  at  five  millions ;  in  the  goat  there  have 
been  found  in  the  same  quantity  of  fluid  eighteen 
millions ;  in  the  rabbit,  three  and  a  half  millions  ; 
in  a  cock,  two  to  three  millions  or  more ;  in 
bony  fishes  seven  hundred  thousand  to  two  millions, 
and  in  various  Elasmobranchs  from  one  hundred 


Chap,  v.]  BLOOD  CORPUSCLES.  183 

and  forty  thousand  to  two  hundred  and  thirty  thou- 
sand (Malassez). 

In  Amphioxus  and  the  Urochordata,  there  are 
no  red  blood  corpuscles,  and,  as  a  rule,  these  are  not 
found  in  what  have  been  called  invertebrates ;  they 
have,  however,  been  observed  in  Solen  (a  Mollusc) ; 
Glycera  (a  Chsetopod)  ;  Amphiporus  (a  Nemertean)  : 
and  Phoronis  (a  Gephyrean). 

In  most  Invertebrates  the  blood  corpuscles 
are  either  limited  to  the  fluid  in  the  body  cavity,  or 
are  found  more  or  less  numerously  represented  in 
the  fluid  contained  in  the  vessels.  In  all  these  cases 
they  are  single  cells,  generally  amoeboid  in  character, 
and  they  vary  considerably  in  size  and  number. 

In  Echiiioderms,  Arthropods,  and  Molluscs, 
the  corpusculated  fluid  is  contained  in  a  system  of 
more  or  less  completely  closed  walls  (vide  infra) ;  in  a 
large  number  of  worms  the  fluid  in  the  vessels  is,  on 
the  other  hand,  said  to  be  non- corpusculated, and,  at  any 
rate  where  corpuscles  are  found,  they  are  often,  as  in 
the  earthworm,  rare,  and  of  small  size  (-g-oVo  ijicn  j 
Lankester).  It  is,  however,  only  quite  recently  that 
observations  have  been  directed  to  the  presence  of 
corpuscles  in  the  blood  of  Annulata,  and  since  then 
they  have  been  observed  in  several  members  of  the 
group  (Eunice,  Nereis). 

BLOOD-VESSELS    AND    HEARTS. 

In  all  the  forms  already  mentioned,  part  of  the 
blood  at  least  is  contained  in  a  system  of  more  or  less 
completely  closed  vessels,  by  means  of  which  it  is 
conveyed  from  part  to  part  of  the  body  ;  these  vessels 
make  up  the  blood- vascular  system.  When  best 
developed  this  system  has,  on  some  parts  of  its  course, 
a  contractile  organ,  by  means  of  which  the  fluid  is 
pumped  along  ;  this  is  the  heart.  In  the  Vertebrata, 
the  vessels  given  off  from  the  heart  (arteries)  do  not, 


184  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

as  in  the  crayfish,  open  into  spaces  in  the  ccelom, 
thence  to  be  taken  up  again  by  the  vessels  which  pass 
to  the  heart  (veins),  but  they  are  conveyed  through 
networks  of  fine  hair-like  vessels  (capillaries),  which 
are  completely  closed.  So,  again,  the  chyle,  or  the 
direct  result  of  the  products  of  digestion,  is  contained 
in  vessels  which  make  their  way  into  the  veins  (the 
more  or  less  completely  closed  lymphatic  system). 

In  the  MoUusca  and  Arthropoda  the  blood- 
vascular  system  is  not  completely  closed  ;  in  other 
words,  the  blood  makes  its  way  from  the  arteries  into 
spaces  or  cavities  in  the  body  cavity,  and  from  these 
incompletely  closed  spaces  it  is  again  taken  up  by  the 
veins  ;  it  is  clear,  therefore,  that  no  closed  or  proper 
lymphatic  system  is  here  needed  ;  the  results  of  the 
process  of  digestion  make  their  way  through  the  walls 
of  the  intestine  directly  into  the  ccelom,  and  thence 
into  the  sinuses,  and  in  this  way  they  replenish  the 
store  of  corpuscles  in  the  blood  of  the  crayfish  or  the 
mussel. 

In  the  Echinodermata  the  blood-vascular  sys- 
tem would  appear  to  be  completely  shut  off  from  the 
ccelorn,  and,  as  it  would  seem  to  be  connected  with  the 
system  of  water  vessels,  it  is,  no  doubt,  diluted  by 
sea-water ;  but  the  fluid  in  the  coelom  contains  char- 
acteristic corpuscles,  some  of  which  are  coloured. 
Curiously  enough,  liEemoglobin  has  been  detected  in 
the  water-vascular  system  of  an  Ophiuroid  (Foettinger). 

In  the  Protozoa  there  is,  of  course,  no  blood, 
but  even  in  the  Amoeba  we  observe  currents  within 
the  protoplasm,  and  some  of  these  are,  no  doubt, 
richer  in  nutriment  than  others,  so  that  by  their 
movement  the  distribution  of  nutritious  material  is 
equalised  over  the  whole  organism  ;  in  Paramcecium 
there  is  an  advance  on  this,  inasmuch  as  in  it  currents 
of  definite  directions  are  to  be  detected.  In  the 
Sponge  the  currents  of  water  that  traverse  its  walls 


chap. v.]       '  BLOOD-VASCULAR  SYSTEM.  185 

bring  in  oxygen  and  food  material.  In  the  Coelen- 
terata  there  are,  as  we  know,  out-growths  of  the 
enteric  cavity,  which,  in  reference  to  their  functions, 
are  spoken  of  as  parts  of  the  gastrovascular 

system  ;  along  these  the  digested  material  and  the 
water  taken  in  by  the  mouth  pass  to  the  different 
cells  which  line  them.  But  the  only  agent  in  the 
propulsion  of  the  material  is  the  pressure  due  to  the 
movements  of  parts  of  the  body. 

In  the  Turfoellaria  we  observe  no  system  of 
vessels ;  the  nutrient  fluid  either  makes  its  way  from 
cell  to  cell,  or  passes  through  the  clefts  and  passages 
in  the  tissues  of  the  body  which  so  often  indicate  all 
that  can  be  seen  of  a  coalom.  In  the  dendrocoelous 
Turbellaria  and  in  the  Trematoda  the  absence  of  a 
system  of  blood-vessels  is,  no  doubt,  largely  made  up 
for  by  the  branches  of  the  gastric  cavity  (see  page  114), 
which  perform  the  same  function  as  the  gastrovascular 
system  of  the  Ccelenterata.  In  the  Tapeworms  the 
only  indication  of  a  system  of  nutrient  vessels  are  the 
delicate  canals  that  lie  internally  to  the  longitudinal 
excretory  vessels,  and  contain  a  homogeneous  plasmatic 
fluid  ;  these  are  the  plasmatic  canals  of  Sommer. 
From  the  Rotatoria,  on  account  of  the  small  size  of 
their  bodies,  a  system  of  blood-vessels  is  wanting. 

The  origin  of  the  closed  system  of  vessels  is 
involved  in  great  obscurity,  but  it  is,  at  any  rate,  to 
be  partly  ascribed  to  the  increase  in  size  of  the 
organism  ;  this  increase  demands  the  possession  of  a 
means  of  providing  for  the  course  of  the  circulating 
medium,  and  affords  us  another  example  of  that  division 
of  labour  which  we  constantly  note  as  we  ascend  the 
scale  of  organisation.  This  explanation  does  not,  at 
first  sight,  appear  to  apply  to  all  of  the  Jtf  emertiiiea, 
for  in  the  smallest  members  of  that  group  we  find  a 
comparatively  elaborate  system  of  not  only  closed  but 
also  contractile  vessels ;  further  investigation  reveals, 


1 86  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

however,  the  important  difference  in  the  function  of 
the  contained  fluid  in  the  lower  as  compared  with  the 
higher  Nemertinea.  In  the  former  haemoglobin  in 
distributed  in  the  nerve  tissue,  but  is  absent  from  the 
blood,  so  that  that  fluid  has  only  nutrient  functions 
in  the  Schizonemertini,  and  not  both  nutrient  and 
respiratory  duties  as  in  the  higher  Hoplonemertini. 
There  are  three  chief  longitudinal  vessels,  two  lateral, 
which  are  connected  with  one  another  at  the  anterior 
end  of  the  body,  and  one  median,  which  is  connected 
with  the  two  lateral  a  little  behind  the  region  of 
the  mouth.  All  these  are  contractile,  but  they  are 
of  the  same  calibre  throughout,  or,  in  other  words, 
there  is  no  special  portion  which  is  enlarged  to  act 
as  a  pumping  organ  or  heart. 

When  we  pass  to  those  higher  forms  of  Worms 
in  which  metarneres  are  developed,  transverse  branches 
or  lateral  vessels  unite  the  median  with  a  now 
ventrally  placed  trunk,  and  some  of  these  lateral 
vessels  become  contractile  (so-called  hearts  of 
Ssenuris  and  others). 

The  dorsal  vessel  (d)  of  such  forms  as,  for  example, 
the  earthworm,  is  retained  in  the  crayfish  as  the  an- 
terior (Fig.  81;  aa')and  posterior  aortse  (pa);  the  trans- 
verse vessels  are  indicated  by  the  short  arteries  at  and 
hp,  which  supply  the  anterior  regions  of  the  body  and 
the  viscera;  one  transverse  vessel  is  still  complete,  and 
forms  the  descending  sternal  artery  (st.a)  which 
opens  into  the  backwardly  and  forwardly  directed 
abdominal  artery  (si.a  ;  iaa)  the  representative  of  the 
ventral  vessel  of  the  earthworm.  In  the  Anodon  (Fig. 
82  c)  the  spaces  or  sinuses  are  much  more  developed, 
and  no  indications  of  a  ventral  vessel  are  now  to  be 
seen ;  the  dorsal  is,  however,  shown  by  the  heart  (H) 
with  its  anterior  and  posterior  aortse  (aa',pa)  ;  while 
the  terminal  parts  of  the  transverse  vessels  become 
enlarged  to  form  the  auricles  of  the  heart  or  ventricle  (a). 


Chap.  V.] 


HEART  OF  ARTHROPODA. 


In  the  fish  (Fig.  82  D)  the  enlargement  for  the  heart  (H)  is 
found  on  the  ventral  vessel;  passing  for  wards  it  branches 
on  either  side  into*  the  branchial  vessels,  and  these  unite 
and  pour  their  blood  into  the  dorsal  (br)  aorta  (da). 

The  blood-vascular  system  of  the  Arthropoda 
is  distinguished  by  the  fact  that  the  blood  which 
comes  to  it  to  be  pumped  through  the  body  does  not 

d 


a  a/ 


Fig.  81. — Diagrams  to  show  the  arrangement  of  the  great  Blood- 
in  the  Earthworm  (A)  ;  the  Crayfish  (B). 


reach  it  directly  by  distinct  vessels  ;  the  heart  is  sur- 
rounded by  an  imperfectly  closed  space,  the 
pericardial  sinus.  When  the  contractile  cham- 
ber which  is  called  the  heart  dilates,  the  blood  in  the 
surrounding  sinus  flows  into  it  through  two  or  more 
spaces  or  holes  in  its  walls.  The  heart  may  be  short, 
as  in  Daphnia,  and  have  only  a  pair  of  orifices ;  or  it 
may  be  greatly  elongated,  as  in  Artemia,  where  there 
are  twenty  pairs ;  or  it  may  be  much  more  compact, 
as  in  the  crayfish,  where  there  are  three  pairs 
of  large  ostia,  one  superior,  one  lateral,  and  one 
inferior,  which  are  guarded  by  valves  that  prevent 


1 88  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  return  of  the  blood  to  the  sinus,  as  well  as  an 
irregular  number  of  smaller  holes.  In  the  occasionally 
parasitic  copepods  (Corycseus)  and  the  degenerated 
Cirripedes  there  is  no  heart. 

The  vessels  arising  from  the  heart  likewise  differ 
considerably  in  their  arrangement  \  in  the  Entomos- 
traca  there  is  an  anterior  artery  only,  which  may  branch 


Fig.  82. — Diagrams  to  show  the  arrangement  of  the  jrreat  Blood-vessels 
in  the  fresh-water  Mussel  (c) ;  and  the  Fish  (D). 


more  or  less  at  its  free  end ;  but  the  greater  part  of 
the  blood  makes  its  way  through  definite  spaces  without 
distinct  enclosing  walls,  the  so-called  lacunae.*  In 
the  Malacostraca  a  posterior  aortic  artery  is  given  off, 
in  addition  to  the  anterior;  and  in  the  crayfish,  for 
example,  we  may  further  distinguish  two  pairs  of 
anteriorly  directed  trunks ;  the  antennary,  which 

*  Our  knowledge  of  the  vascular  system  of  Arthropoda,  or 
Molluscs,  is  in  an  unsatisfactory  condition;  the  "lacunar  con- 
nection between  the  arteries  and  veins,  which  is  confidently  de- 
scribed and  discussed  by  all  zoologists,  has  never  yet  been  demon- 
strated to  exist  in  a  manner  satisfying  the  requirements  of  modern 
histology  "  (Lankester). 


chap. v.i  HEART  OF  ARTHROPODA.  189 

supply  the  front  part  of  the  head,  and  the  hepatic, 
which  go  to  the  chief  viscera.  The  posterior  artery 
rims  backward  along  the  dorsal  surface  of  the  tail,  and 
gives  off  on  its  course  a  downwardly-directed  vessel 
(sternal  artery),  which  on  reaching  the  ventral  region 
divides  into  an  anterior  and  a  posterior  abdominal 
artery.  As  these  several  vessels  ramify,  they  break 
up  into  smaller  vessels,  and  then  finally  open  into 
spaces  among  the  various  organs  of  the  body.  The 
largest  and  most  important  of  these  is  the  great 
sternal  sinus,  which  lies  in  the  region  of  the 
entrance  to  the  gills,  into  the  spaces  in  which  the 
blood  passes  to  receive  a  fresh  supply  of  oxygen  (see 
page  224) ;  thence  the  blood  returns  by  the  branchio- 
cardiac  veins  or  canals  to  the  pericardial  sinus,  to 
again  pass  into  the  heart,  and  resume  its  journey 
through  the  body. 

It  will  be  observed  that  the  blood  goes  to  the 
organs  of  the  body  from  the  heart  before  it  goes  to 
the  gills.  Such  a  heart  is  known  as  a  systemic 
heart,  in  contradistinction  to  the  branchial  heart  of 
fishes,  for  example,  in  which  the  blood  pumped  from 
the  heart  goes  firstly  to  the  gills,  and  secondly  to  the 
other  organs  of  the  body. 

The  vascular  system  of  Peripatus  is  described 
by  Balfour  as  consisting  of  a  dorsal  vessel  shut  off 
from  the  body  cavity  by  a  continuation  of  the  endo- 
thelial  lining  of  the  latter.  It  has  definite  walls, 
but  it  is  not  clear  whether  they  are  muscular.  It  ex- 
tends from  near  the  hinder  end  of  the  body  to  the 
head,  and  is  largest  behind.  Between  the  skin  and 
the  outer  layer  of  muscles  there  is  a  very  delicate 
ventral  vessel. 

In  the  Myriopoda  the  heart  extends  through 
the  whole  of  the  body,  and  is  made  up  of  a  number  of 
chambers  separated  from  one  another  by  valves  pro- 
vided with  orifices  for  the  entrance  of  the  venous 


190  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

blood,  and  giving  off  in  regular  metameric  fashion  a 
pair  of  arterial  vessels.  Anteriorly  the  cardiac 
trunk  is  continuous  with  a  vessel  which  lies  on  the 
upper  surface  of  the  ventrally  placed  nerve  cord. 

In  the  Hexapoda  the  Cardiac  tube  is  confined 
to  the  abdominal  region  of  the  body,  and  there  is  a 
smaller  number  of  separate  chambers.  The  anterior 
tube  is  known  as  the  aorta ;  its  further  ramifications 
are  not  known.  In  them  and  in  the  Myriopoda 
the  pericardiac  sinus  has  connected  with  it  several 
pairs  of  ordinarily  fan-shaped  muscles  (alse  cordis) ; 
as  these  are  not  directly  attached  to  the  cardiac  tube, 
they  cannot,  as  has  been  sometimes  supposed,  have 
any  function  in  the  way  of  dilating  the  heart ;  it  is 
probable,  however,  that  they  enlarge  the  extent  of  the 
pericardiac  sinus,  and  thereby  assist  or  accelerate  the 
flow  of  blood  into  it.  The  inner  lining  of  the  heart  is 
elastic,  and  the  outer  muscular  coat  does  not  contract 
simultaneously,  but  from  behind  forwards  (Lowne). 

The  blood-vascular  system  of  the  Araclmida,  as 
represented  by  Limulus  and  Scorpio,  is  more  com- 
plete than  that  of  any  other  Arthropod  ;  fine  vessels 
given  off  from  the  arteries  form  a  true  capillary 
system,  and  the  veins  are  definite  and  distinct.  The 
heart  is  elongated,  and  consists  of  eight  chambers, 
each  provided  with  a  pair  of  apertures  guarded  by 
valves ;  it  is  continued  forwards  into  an  anterior,  and, 
in  the  scorpion,  backwards  into  a  posterior  aorta.  In 
the  scorpion  each  cardiac  chamber  gives  off  an  artery 
on  either  side,  and  several  pairs  are  given  off  from  the 
posterior  aorta.  Anteriorly,  the  aorta  forms  a  collar 
round  the  cerebral  nerve-mass,  and  is  continued  into 
a  ventral  artery  which  lies  above  the  ventral  nerve- 
cord  ;  this  artery  is  intimately  connected  with  the 
nerve-chain  in  the  scorpion,  and  in  Limulus  it  absor 
lutely  surrounds  it. 

In  the  spiders  and  other  Arachnids  the  number  of 


Chap,  v.]  HEART  OF  MOLLUSCA.  191 

cardiac  chambers  is  'reduced,  and  in  the  mites  appear 
to  be  altogether  absent. 

The  circulatory  system  of  the  Mollusca  presents 
a  remarkable  difference  from  that  of  the  Arthropoda, 
in  so  far  as  the  blood  never  passes  into  the  peri- 
cardiac  sinus.  The  heart  is  again  formed  from  part 
of  the  dorsal  vessel,  and  in  the  least  modified  forms, 
or  such  as  still  present  a  bilateral  symmetry,  a  pair, 
or  two  pairs  (Nautilus),  of  transverse  vessels  open 
directly  into  the  central  or  axial  portion  of  the  heart ; 
the  ends  of  these  vessels  nearest  to  the  axial  portion 
are  enlarged  in  size  and  modified  to  form  auricles, 
while  the  altered  part  of  the  dorsal  trunk  serves  as  a 
ventricle,  from  which  the  blood  passes  forwards  by 
an  anterior,  and  backwards  by  a  posterior,  aorta. 

A  simple  arrangement  of  this  kind  is  well  seen  in 
the  mussel  (Anodon),  or  in  the  squid  (Loligo).  What 
is  probably  a  still  more  primitive  arrangement  is 
presented  by  the  Nautilus,  in  which  there  are  two 
pairs  of  transverse  vessels,  and  therefore  two  pairs  of 
auricles.  In  the  Octopus  the  aortic  vessel,  which  in 
the  mussel  was  directed  backwards,  now  takes  a 
forward  course,  or  at  first  runs  parallel  to  the  true 
anterior  aorta ;  in  those  Gastropods  that  have  suffered 
a  more  or  less  well-marked  torsion  of  the  chief  viscera 
(see  page  81),  there  is  but  a  single  auricle,  and  the  great 
vessel  arising  from  the  front  end  of  the  ventricle  early 
divides  into  two  branches ;  of  these  one,  like  the 
anterior  aorta  of  the  mussel,  supplies  the  front  end  of 
the  body,  while  the  other  is,  in  like  manner,  distributed 
to  the  chief  viscera. 

The  circulation  is,  to  a  large  extent,  effected 
in  a  manner  which  has  been  called  lacunar ;  but, 
as  has  been  already  pointed  out,  our  knowledge  of 
these  lacunae  is  in  a  very  elementary  condition.  The 
statement  that  water  is  taken  up  into  the  blood- 
vascular  system  by  pores  in  the  foot  does  not  appear 


192  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

to  rest  on  good  foundation  and  is  disproved  by 
a  number  of  observations.  The  system  of  vessels 
is  better  developed  in  the  Cephalopoda  than  in  other 
Molluscs  ;  the  definite  arteries  are  more  numerous, 
and  their  finer  ramifications  are  more  distinctly  capil- 
lary in  nature.  The  contractile  power  of  the  gills  is, 
no  doubt,  of  some  aid  in  the  propulsion  of  the 
blood  ;  the  walls  of  the  vessels  connected  with  them 
are,  in  the  two-gilled  Cephalopods,  provided  with 
muscles,  and  the  name  of  branchial  heart  has 
been  given  to  the  enlarged  portion  of  these  arteries. 

The  three  great  divisions  of  the  Chord  ates  must 
be  dealt  with  separately.  The  Urochordata  are 
remarkable  for  an  arrangement  which,  though  not 
unique  in  the  animal  kingdom  (for  it  has  been  ob- 
served also  in  the  embryos  of  certain  gastropods),  is  a 
very  striking  characteristic,  and  most  instructive  pheno- 
menon. It  has  been  observed  that  in  them  the  pul- 
sations of  the  heart,  having  resulted  in  the  movement 
of  the  blood  current  in  a  forward  direction,  are,  after 
a  pause,  reversed,  so  that  the  blood  flows  backwards 
instead  of  forwards.  After  this  backward  movement 
has  obtained  for  a  time  there  is  another  pause,  and 
this  is  succeeded  by  a  forward  movement  of  the 
blood. 

The  heart  of  Tunicates  has  the  shape  of  a  tubular 
or  fusiform  sac,  and  gives  off  a  large  vessel  at  either 
end.  In  distinction  to  the  forms  already  considered, 
and  in  agreement  with  what  obtains  in -the  Verte- 
brata,  the  heart  appears  to  be  an  enlargement  of  a 
ventral,  and  not  of  a  dorsal,  vessel.  The  trunks 
which  arise  from  it  break  up  into  vessels  which, 
according  to  the  area  of  their  distribution,  may  be 
grouped  as  branchio-cardiac,  cardio-splanchnic,  or 
splancho-branchial ;  and,  in  addition  to  these,  there 
are  a  number  of  anastomosing  vessels  in  the  test. 
When  the  heart  contracts  from  behind  forwards 


Chap,  v.]  HEART  OF  CHORD  ATA.  193 

(that  is,  from  its  ventral  towards  its  dorsal  end)  it 
contains  almost  pure  arterial  blood,  and  may,  there- 
fore, be  regarded  as  a  systemic  heart ;  on  the  other 
hand,  when  the  contractions  are  reversed  in  direc- 
tion, the  blood  is  nearly  all  impure,  and,  as  it 
largely  passes  to  the  gills,  the  heart  may  now  be  said 
to  be  branchial  or  respiratory. 

The  cardiac  tube  is  sometimes  constricted  at 
various  points,  but  is  never  divided  into  distinct 
chambers.  The  most  remarkable  condition  is  pre- 
sented, not  only  among  the  Tunicata,  but  among  all 
known  animals,  by  Appendicularia  furcata.  In 
it  the  heart  consists  of  but  two  cells,  which  are  con- 
nected with  one  another  by  from  twelve  to  twenty- 
five  processes,  between  which  there  are  open  spaces. 

In  the  Ceplialocliordaifa  we  find  an  arrange- 
ment which  reminds  us  of  what  obtains  in  the 
Anmilata,  inasmuch  as  there  is  no  centralised  con- 
tractile heart,  but  the  blood  is  only  moved  forwards 
by  the  contractility  of  some  of  the  great  vessels.  The 
vessel  of  largest  calibre  is  found  in  the  neighbourhood 
of  the  anterior  gill- clefts ;  into  this  the  blood  passes 
from  the  gills,  and  from  it  it  goes  into  a  vessel 
which  is  connected  with  the  right  of  the  two  so-called 
aortic  trunks.  These  two  trunks  unite  with  one 
another  behind  the  branchial  area,  and  form  a  single 
dorsal  vessel,  or  "  aorta,"  which  extends  backwards 
along  the  body ;  at  the  hinder  end  it  is  continuous 
with  a  ventral  vessel,  which,  on.  its  way  forwards  to 
the  gills,  gives  off  some  branches  to  the  rudimentary 
liver  (see  page  161),  and  so  forms  a  kind  of  rudimentary 
portal  system.  (See  page  206.)  The  blood  from  the  liver 
returns  to  the  great  ventral  trunk,  and,  with  the  rest, 
makes  its  way  to  the  gills  to  receive  a  fresh  supply  of 
oxygen.  The  branchial  vessels  form  dilatations  on 
their  course  (bulbilli),  and  the  contained  blood  either 
makes  its  way  directly  into  one  of  the  aortae,  or  first 
N— 16 


194  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

passes  through  the  already-mentioned  anterior  en- 
largement. 

A  very  definite  system  obtains  throughout  the 
Verteforata ;  there  is  always  a  centralised  ventrally 
placed  heart,  which  consists  of  at  least  two  chambers, 
an  auricle  and  a  ventricle;  from  the  latter,  one,  or  a 
pair,  or  several  pairs  of  arterial  vessels  (aortic 
arches)  are  given  off;  these  divide  into  smaller  and 
smaller  arteries,  which  end  in  the  capillaries  found 
in  all  organs  and  parts  of  the  body ;  the  capillaries 
pour  their  blood  into  the  small  veins,  and  the  small 
veins  into  larger  ones,  three,  or  less  than  three,  of 
which  open  into  the  auricular  region  of  the  heart. 

Where  respiration  is  effected  by  gills  the  blood 
goes  through  the  aortic  arches  directly  to  these  organs, 
distributes  itself  in  the  fine  gill-capillaries,  and  then, 
re-collecting,  distributes  itself  through  the  body  ;  the 
heart,  therefore,  of  a  lowly  Vertebrate  is  branchial, 
and  not  systemic  like  that  of  the  gill-bearing  cray- 
fish ;  when  lung-like  structures  are  superadded  to  the 
gills,  the  heart  becomes  incompletely  divided  into  two 
halves,  and  where  lungs  altogether  take  the  place  of 
gills  there  is  a  tendency,  which  in  the  higher  forms 
becomes  an  accomplished  fact,  for  the  heart  to  become 
divided  into  two  separate  parts ;  one  of  these,  that 
on  the  right  side,  collects  the  blood  from  the  body, 
and  sends  it  to  the  respiratory  organ,  or  acts  the 
part  of  a  branchial  heart,  while  the  other  (left 
side)  receives  the  blood  from  the  lungs  and  pumps 
it  into  the  body,  or,  in  other  words,  acts  as  a  systemic 
heart. 

The  heart  is  placed  in  a  membranous  pouch  or 
bag,  the  pericardium,  and  ordinarily  hangs  freely 
in  it,  though  sometimes,  as  in  the  eel,  the  heart  is 
attached  to  it  by  fibrous  bands ;  the  successive  cham- 
bers are  separated  from  one  another  by  valves,  and  the 
ventricle  is  likewise  separated  from  the  aortic  system 


chap,  v.]  HEART  OF  VERTEBRATES.  195 

by  similar  structures ;  these  are  numerous  in  the 
lower,  but  reduced  to  a  single  set  in  the  higher  forms. 

The  blood,  on  returning  from  the  body  to  the  heart, 
is  in  the  lower  Vertebrata  collected  into  an  enlarge- 
ment of  the  venous  system  which  is  known  as  the 
sinus  venosus,  and  the  walls  of  this  chamber  are, 
like  those  of  the  auricle  and  ventricle,  contractile. 
Contractility,  then,  is  not,  as  in  the  higher  members  of 
the  group,  confined  to  the  centralised  heart ;  this  is 
well  illustrated,  on  the  one  hand,  by  the  Myxinoids,  in 
which  the  portal  vein,  and  by  the  eel  (McWilliam), 
in  which  the  terminations  of  the  jugular  veins,  are 
contractile  ;  and  on  the  other,  by  the  Elasmobrarich 
fishes,  the  Dipnoi,  and  the  Amphibia,  in  which  the 
basal  portion  of  the  aortic  system  (conus  arteriosus) 
is  also  contractile  (Fig.  83).  Here,  as  elsewhere,  we 
have  evidence  of  gradation  in  the  division  of  labour. 

The  auricle,  which  is  a  single  thin-walled  sac  in 
most  fishes,  becomes  more  or  less  divided  into  two  in 
the  Dipnoi;  along  the  left-hand  division  of  the  heart 
there  flows,  in  addition  to  the  blood  from  the  veins, 
that  which  has  been  returned  from  the  rudimentary 
lung  (see  page  232) ;  along  the  right  side  the  rest,  or 
purely  venous,  blood  passes.  Now,  the  arterial  cone 
or  trunk  arises  rather  from  the  left  than  from  the  right 
side  of  the  ventricle,  which  is  incompletely  divided 
into  two  halves,  so  that  the  blood  which  first  leaves  it 
is  the  blood  from  the  left  auricle  ;  this  will,  of  course, 
go  to  the  farthest  gill  vessels,  or  those  of  the  first  and 
second  arch  ;  the  last  arch  of  all,  the  fourth,  will,  of 
course,  receive  the  most  impure  or  venous  blood,  and 
it  is  the  one  which,  in  Ceratodus,  sends  off  a  trunk 
to  the  lung. 

This  division  of  the  auricle,  which  is  hinted  at 
even  in  Chimsera  (Lankester),  becomes  complete  in 
the  forms  which  constantly  breathe  air  by  means  of 
distinct  lungs,  and  the  sinus  venosus,  which  brings  the 


1*96  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


blood  from  the  body  generally,  opens  into  tlie  right, 
and  the  lung  vein  into  the  left  compartment ;  in  all 
the  higher  forms,  however,  the  two  halves  are  more 
or  less  connected  during  embryonic  life,  and  just  as 
the  tadpole  has  only  a  single  auricle,  so  even  in  man 
there  is  a  communication  between 
the  right  and  left  sides  (foramen 
ovale  of  the  interauricular  sep- 
tum), which,  in  exceptional  cases, 
remains  open  in  the  adult  con- 
dition, and  thereby  produces  the 
affection  known  as  cyanosis.  In 
the  Mammalia  the  sinus  venosus, 
which  in  Ceratodus  (Lankester), 
though  not  in  most  lower  Verte 
brata,  is  not  sharply  separated  off 
from  the  auricle,  becomes,  when 
foetal  development  is  over,  com- 
pletely merged  into  the  right 
auricle  (atrium  or  sinus  venosus  of 
human  anatomy). 

The  ventricle  remains  an  un- 
divided chamber  throughout  the 
Amphibia  and  all  Reptiles  except 
the  Orocodilia,  so  that  it  is  clear 
that  the  presence  of  two  ventricles 
in  Crocodiles,  Birds,  and  Mammals, 
is  not  a  homogenetic,  but  a  homo- 
plastic  arrangement.  (Compare  page  12.)  An  in- 
teresting example  of  the  "  falsification  of  the  embryo- 
logical  record,"  is  afforded  by  the  development  of  the 
ventricles,  inasmuch  as  in  those  forms  where  they  are 
distinct,  they  become  so  before  and  not  after  the 
auricles ;  it  is  a  case  of  what  Haeckel  calls  cenogeny, 
and  is,  no  doubt,  dependent  on  the  requirements  of 
the  organism. 

The   ventricular  is  separated  from  the  auricular 


Fig.   83.  -  Heart 
Squatina. 


of 


L,  Auricle;  B,  arterial 
cone;  aaa,  branchial 
arterios ;  o,  orifice  of 
ventricle,  v.  (After 
Gegenhaur.) 


Chap.  V.] 


HEART  OF  VERTEBRATES. 


197 


portion  of  the  heart  by  membranous  valves,  just  as 

the   auricle   is    shut  off  from   the   venous    sinus   by 

similar    structures ;    these    are    in    fishes    ordinarily, 

though  by  no  means  always,  two  in  number;  and  their 

function  is  clearly  to  close  the  way  back  into  the 

venous   system,   and  thereby  to  aid    in   forcing   the 

blood  forwards,  on  the  contraction 

of  the  walls  of  the  cavity.     In  the  Tft 

Amphibia  (Fig.  84)  the  two  valves 

are  fibrous,  and  are  connected  by 

fibres  with  the  wall  of  the  ventricle, 

so  that  when  this  part  of  the  organ 

contracts  it  draws  down  the  valves  ; 

when  the  auricular  chamber  (atrium) 

becomes  divided,  each  opening  into 

the     ventricle     is     provided     with 

valves. 

In  the  turtle  the  auriculo- ven- 
tricular valves  are  formed  by  the 
development  of  the  ventricular  edge 
of  the  auricular  septum  into  two 
folds,  which,  on  the  contraction  of 
the  ventricle,  meet  a  ridge  on  the 
correspondingly  opposite  side  of  the 
ventricle ;  in  the  crocodile  these 
folds  are  distinct  valves.  In  the  Bird 
we  find  a  difference  between  the  valves  of  the  right 
and  left  side ;  on  the  right  two  folds  of  muscular 
tissue,  close  together  at  their  auricular  end,  diverge 
from  one  another,  and  extend  far  down  into  the  right 
ventricle ;  on  the  left  the  muscular  is  largely  replaced 
by  fibrous  tissue  which  gives  off  fine  tendons  (chordae 
teiidinese)  to  projecting  muscular  processes  of  the 
wall  of  the  ventricle  (musculi  papillares)  (Fig. 
85) ;  these  tendinous  chords  are  grouped  into 
three  masses,  and  there  are  three  muscular  elevations. 

Though  it  is  possible  to  derive  the  arrangement  of 


Fig.  84.— Heart  of  the 
edible  Frog  (Rana 
esculenta),  to  sho* 
the  auricle  and  ve 
tricle  opened  from 
the  left  side. 

s,  Septum  atriorum  ;  la, 
left  auricle  ;  ra,  right 
auricle  ;  w,  auriculo- 
ventricula.-  valves ;  o, 
orifice  of  arterial  cone 
or  trunk  :  v.  ventricle. 
(After  Ecker.) 


198  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


valves  which  is  found  in  the  bird  from  that  which 
obtains  in  reptiles,  no  such  comparison  is  possible  in 
the  case  of  the  mammalian  valves ;  in  most  of  the 
Mammalia  we  find  that  the  valves  are  membranous 
and  not  fleshy,  and  that  there  are  three  (tricuspid) 
valves  in  the  right  auriculo-ventricular  orifice,  and 

two  in  the  left  (mitral 
valves) ;  as  in  the  left 
side  of  the  heart  of  the 
bird,  there  are  chorda? 
tendiiiese  and  musculi 
papillares,  but  the  num- 
ber of  these  is  not  con- 
fined to  three.  In  the 
rabbit  the  valve  of  the 
right  side  is  continuous, 
and  not  produced  into 
three  cusps.  In.  the 
Ornithorhynchus,  the 
valve  of  the  right  side 
is,  as  in  the  Sauropsida, 
muscular  membranous 
tissue  being  only  com- 
paratively feebly  de- 
veloped in  it. 

As  in  the  other  parts  of  the  heart,  so  in  the 
ventricle,  we  find  an  instructive  series  of  gradations. 
Single  and  undivided  in  all  Fishes  except  the  Dipnoi, 
it  is  always  much  stronger,  and  has  much  thicker 
walls,  which  are  largely  composed  of  muscular  tissue, 
than  has  the  auricular  region ;  for  while  the  office  of 
the  contraction  of  the  auricle  is  merely  to  drive  the 
blood  into  the  adjoining  chamber,  the  ventricle  has 
the  chief  part  to  play  in  forcing  it  through  the  body. 
In  the  Teleostei  and  some  Ganoids  the  wall  of  the 
ventricle  is  arranged  in  two  layers,  between  which  is 
a  lymphatic  space. 


Fig.  85.— Left  auriculo-veutricular 
valve  of  the  Swan,  showing  the 
chordae  tendinese  and  the  musculi 
papillares  (pp).  (After  Wieder- 
sheim.) 


Chap,  v.]  HEART  OF  REPTILES.  199 

An  indication  of  a  division  of  the  ventricle  into 
two  parts  is  seen  in  the  Dipnoi,  but  in  the  Amphibia 
there  is  no  septum;  in  the  uni- ventricular  Reptiles 
(that  is,  in  all  but  Crocodiles)  there  is  no  complete 
division  of  the  cavity,  but  the  muscular  walls  form 
internal  projections  which  are  functionally  of  some 
importance ;  the  most  valuable  of  these  is  the 
prominent  fold  which  lies  just  beneath  the  entrance  to 
the  pulmonary  artery,  and  almost  separates  off  the 
part  of  the  ventricular  cavity  which  lies  beneath  it, 
from  the  rest  of  the  cardiac  chamber ;  in  consequence 
of  this  being  the  region  whence  blood  passes  directly 
to  the  lungs  by  the  pulmonary  arteries  (PA),  it  is 
known  as  the  cavtim  piiliiioiialc  (Fig.  86 ;  Cp\ 
and  it  occupies  the  right  extremity  of  the  heart.  As 
the  ventricle  contracts,  the  blood  in  this  cavum  is 
forced  into  the  pulmonary  artery,  and  as  it  is  the 
blood  which  has  entered  the  ventricle  from  the  right 
auricle  (RA),  it  is,  of  course,  venous  blood,  or  blood 
that  requires  oxygenation.  As  the  contraction  con- 
tinues, the  wall  of  the  ventricle  and  the  edge  of  the 
septum  are  brought  closer  together,  so  that  the  blood 
which  is  the  last  to  leave  the  cavity  is  prevented  from 
making  its  way  into  the  cavum  pulnionale ;  this  blood 
is  that  from  the  left  side  of  the  heart,  that  is,  from 
the  left  auricle  (LA)  ;  in  other  words,  it  is  blood 
which  has  just  returned  from  the  lungs,  and  requires 
no  further  oxydation,  and  it  passes  altogether  into 
the  systemic  aortse.  An  inspection  of  the  figure  of 
the  heart  will,  however,  show  that  some  venous  blood 
must  pass  into  the  same  vessels,  so  that  the  blood 
in  the  systemic  vessels  of  the  tortoise  is  not  pure 
arterial  blood,  but  is  a  mixture  of  partly  oxygenated 
blood  and  of  blood  that  has  already  made  a  passage 
through  the  body. 

In  the  Crocodilia,  Birds,  and  Mammals  there  is  a 
complete  interventricular  septum,  so  that  within  the 


200   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

heart  itself  the  freshly  oxygenated  or  arterial,  and  the 
impure  or  venous  blood  never  commingle  ;  in  the 
Crocodile,  where  an  aortic  arch  is  in  communication 
with  each  half  of  the  ventricle,  the  arterial  and 
venous  blood  commingle  outside  the  heart  at  the 
point  of  union  of  their  two  vessels  (the  so-called 
foramen  Panizzse)  ;  in  birds  and  mammals  there  is  but 


Ji.AO. 


Fig.  86.— Diagram  of  the  Ventricle  and  connected  parts  in  the  Turtle  ; 
showirg  the  transversely  elongated  ventricle,  with  the  right  and 
left  auricles  (RA,  LA)  lying  towards  the  left,  the  auriculo- 
veutricular  valves  (uu)  formed  by  the  inter-auricular  and  septum. 

RA,  Right  auricle;  LA,  left  auricle;  »,  the  right;  t/,  the  left  median  auriculo- 
ventricular  valves;  SL.,  arrow  showing  the  course  of  the  blood  in  the  left 
aorta;  t,  arrow  showing  the  course  of  the  blood  in  the  right  aorta ;  x,  arrow 
showing  the  course  taken  by  the  blood  from  the  left ;  and  to,  from  the  right 
auricle  into  the  ventricle  ;  y.  showing  the  course  of  the  blood  from  the  cavuiu 
venosum  into  the  cavum  pulmonale  ;  z,  from  the  latter  into  the  pulmonary 
artery  ;  a,  the  incomplete  septum  marking  off  the  cavum  pulmonale  (cp) ;  PA, 
pulmonary  artery  ;  RAO,  LAO,  right  and  left  aortoe  ;  t>,  cavum  venosum.  (After 
Huxley.) 

a  single  aortic  arch,  which  arises  from  the  left  ven- 
tricle, and  the  blood  from  the  right  never,  therefore, 
passes  into  the  aorta. 

The  differences  between  the  arrangement  of  the 
auriculo-ventricularvalves  have  been  already  described, 
and  we  now  need  only  point  out  that  there  must  be  a 
difference  in  the  way  in  which  these  valves  perform 
their  office;  in  the  Sauroid  they  are  muscular,  and 
therefore  actively  close  the  entrance  to  the  auricles  by 
contracting  when  the  ventricles  contract,  while  in  the 
majority  of  Mammals  the  membranous  flaps  are 


Chap. v.]  HEART  OF  VERTEBRATES.  201 

floated  upwards  by  the  pressure  of  the  blood  contained 
in  the  ventricles,  when  acted  on  by  the  contraction  of 
the  walls  of  these  cavities. 

The  peculiarities  of  the  muscular  tissue  of  the 
heart  of  vertebrates  are  dealt  with  in  works  on  human 
or  general  physiology  ;  but  it  must  be  pointed  out  that 
this  tissue  is  remarkable  for  the  possession  of  hsemo- 
globin  ;  that,  under  appropriate  conditions  of  warmth 
and  moisture,  the  heart  of  a  frog  or  a  tortoise  will, 
after  removal  from  the  bodv,  continue  to  beat  auto- 
matically for  a  number  of  hours ;  and  that  minute 
threads  of  the  tissue  from  certain  regions  possess  the 
same  peculiarity. 

In  the  Mammalia  the  muscular  tissue  of  the  heart 
is  supplied  with  proper  blood-vessels  (the  coronary 
arteries),  which  arise  directly  from  the  aorta,  and 
after  branching  elaborately,  unite  into  the  coronary 
veins  which  open  into  the  right  auricle.  In  some, 
especially  Ungulates,  a  bone,  which  in  the  ox  may  be 
as  much  as  an  inch  in  length,  is  developed  in  the  walls 
of  the  heart.  An  analogous  development  obtains  in 
the  penis,  where  a  bone  is  sometimes  present. 

The  ventricular  portion  of  the  heart  gives  oft1 
vessels  which  are  known  as  the  arteries ;  in  the  least 
modified  Fishes,  and  in  the  Ganoids,  the  common  trunk 
(conns or  trimcus  arteriosus)  is,  like  the  venous 
sinus,  contractile,  but  in  the  bony  fishes  this  con- 
tractile power  is  altogether  lost,  and  the  bulbus 
aorta',  as  it  is  there  called,  becomes  simpler  in  con- 
struction, while  the  valves  which  prevent  the  blood 
from  flowing  backwards  are  ordinarily  reduced  to 
two ;  the  loss  of  the  valves  is  clearly  correlated  with 
the  loss  of  contractility,  for  there  is  not  in  the  walls 
of  this  bulb  any  means  by  which  the  column  of 
blood  can  be  compressed,  and  thereby  tend  to  be  driven 
back  into  the  ventricle.  Where  contractility  is,  on 
the  other  hand,  retained,  we  find  three  (dog-fish)  or 


202   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


more  (in  Lepidostens  eight  or  nine)  longitudinal  rows 
of  pocket-like  valves  ;  in  Lepidostens  there  are  four 
well-developed  and  four  smaller 
valves  in  each  of  the  nine  planes, 
so  that  were  they  all  complete  there 
would  be  as  many  as  seventy-two. 

Among  the  Dipnoi,  Ceratodus 
has  one  or  more  rows  of  well- 
developed  pocket  valves,  but  the 
fact  that  the  number  is  inconstant 
shows  that  a  change  is  impending  ; 
such  a  change  is  found  in  Protop- 
terus,  where  the  valves  are  few  in 
number  and  minute  in  size,  while 
their  place  is  taken  by  a  longitu- 
dinal fold,  which  extends  down  the 
greater  part  of  the  cone,  and  very 
possibly  owes  its  origin  to  a  fusion 
of  a  row  of  valves.  By  means  of 
the  valve  the  cone  is  divided  into  a 
right  and  a  left  half,  and  the  blood 
that  has  just  returned  from  the 
body  is  now  carried  to  the  third 
and  fourth  arches,  the  latter  of 
which  gives  off  a  large  pulmonary 
artery,  or  vessel  which  goes  direct 
to  the  lungs. 

The  essential  parts  of  this  ar- 


g. 

the  Arterial  Circu- 
lation    in     Fishes. 
(AfterWiedersheim.) 


rangement  are  seen  among  some  of 


the  Amphibia  ;  but,  as  may  be  sup- 
posed from  what  has  already  been 
said  of  the  arrangement  of  the  ventricle  in  the  lower 
Reptilia,  no  functionally  independent  arterial  cone  is 
to  be  observed  in  them ;  nor  is  it  seen  in  the  adults  of 
the  higher  Vertebrates,  though  even  there  it  is  at  first 
a  distinct  part  of  the  heart,  and  is  undivided  both 
within  and  without. 


Chap,  v.]         ARTERIES  OF  VERTEBRATES.  203 

From  this  cone  or  bulb  cf  the  heart  there  pro- 
ceeds a  vessel  which  soon  breaks  up  into  a  number 
of  arches  (Fig.  87);  in  Fishes  the  number  of  these 
is  in  correspondence  with  that  of  the  gill  clefts. 
Within  the  substance  of  the  gill  plate  the  artery 
(branchial  artery)  breaks  up  into  a  plexus  of 
tine  capillaries,  and  these  become  collected  into  a 
common  trunk  on  either  side  which  passes  forwards 
to  the  brain  and  backwards  to  the  rest  of  the 
body  ;  behind  the  heart,  the  two  trunks  unite  into 
a  single  median  and  dorsal  aorta,  whence  vessels 
(arteries)  are  given  off  to  the  different  organs  and 
regions  of  the  body. 

When,  as  in  the  Dipnoi,  a  pair  of  lung  sacs 
become  developed,  one  of  the  branchial  vessels  (the 
fourth)  gives  off  on  its  way  from  the  gills  a  large 
trunk  which  passes  directly  to  the  lungs,  whence 
the  blood  is  returned  directly  to  the  left  side  of 
the  heart.  When  the  gills  are  lost  altogether  the 
branchial  capillaries  lose  their  function,  and,  for  the 
greatest  part,  become  aborted,  though  the  frog  re- 
tains in  its  so-called  carotid  gland  the  plexiform 
arrangement  of  the  capillaries  which  was  of  use  to 
it  in  its  gill-bearing  tadpole  stage.  As  the  arterial 
cone  is  retained  by  the  Amphibia,  the  general  re- 
lation of  the  great  vessels  to  the  ventricle  is  the  same 
as  in  Fishes,  and  the  only  differences  that  obtain  are 
such  as  are  due  to  the  differences  in  function  of  differ- 
ent vessels,  which  influence  their  size  and  distribution. 

In  the  Beptilia,  as  has  been  already  explained, 
the  orifices  of  the  great  vessels,  which  are  ordinarily 
guarded  by  merely  two  semilunar  valves,  are  brought 
into  closer  relation  with  certain  parts  of  the  ventricle; 
the  arterial  cone  (Fig.  88 ;  tr)  becomes  shorter,  and  is 
divided  internally  by  septa. 

In  the  lizard  (Fig.  88)  three  arches  arise  from 
the  heart ;  the  two  anterior  are  aortic,  the  third 


204  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


pulmonary.  While  three  arches  arise  from  the 
heart  in  many  reptiles,  only  two  are  directly  given 
off  in  Ophidia,  one  of  which  is  aortic  and  one  pul- 
monary. In  the  bird  and  mammal  this  reduction  is 

carried      still 

</  n  farther,       for 

in  them  the 
aortic  trunk 
is  single 
throughout 
its  whole  ex- 
tent, the  left 
half  being  re- 
duced in  the 
former,  and 
the  right  in 
the  latter. 
These  reduc- 
tions are  best 
explained  by 
a  study  of  the 
figures  of 
Rathke  (Fijj. 
89). 

The     fol- 
lowing      are 
the      names 
and    areas  of 
distribution  of  the  more  important  arteries  : 

1.  Carotids. — These  may  be  double,  when  they 
are  known  as  external  and  internal,  or  one  or  other 
may  be  reduced  or  disappear  ;  in  some  Fishes  the 
carotids  are  not  direct  continuations  of  branchial 
vessels,  but  the  latter  first  unite  to  form  a  cireulus 
cephaliciBS,  by  means  of  which  the  supply  of  blood 
to  the  head  is  the  better  regulated  ;  they  supply  the 
head  and  neck. 


-As 


A(7 


Fig.  88.     Heart  of  Laoerta  inuralis. 

A,  Auricles ;  v,  ventricle;  tr,  arterial  cone  (or  triincus 
anonym  us):  1,  2,  first  and  second  arterial  arches; 
RA,  root  of  aorta;  AC,  aorta;  AS  AS',  subclavian 
arteries;  Ap,  pulmonary  artery  ;  rp,  pulmonary  vein; 
j, jugular ;  vs.  subrlavian  veins;  ci,  vena  cava  in- 
ferior :  these  last  three  pass  into  the  sinus  venosus 
(8)  which  lies  beneath  the  right  auricle.  (At ter  Wie- 
dersbciiuj 


Chap.  V. 


ARTERIAL  ARCHES. 


205 


2.  Subclavian.— This  is  given  off  from  the  aortic 
arch,  and  supplies  the  fore-limb  of  either  side. 

3.  Pulmonary    artery    (see  Fig.    88),    which 
supplies   the    lungs ;    this    in  the    Amphibia,    where 


Fig  89. — Diagrams  to  show  the  Metamorphosis  of  the  Arterial  Arches. 

A,  Lizard  ;  B,  Snake  ;  c,  Bird  ;  u.  Mammal.  The  five  primitive  arches  are  shown 
from  below.  The  first  and  second  arches  are  always  obliterated  and  their 
trunks  carried  on  as  the  internal  («)  and  external  (6)  carotids.  These  are  both 
continuous  with  the  third  arch  (c)  which  forms  the  common  carotid,  and  in 
A  arises  directly  from  the  arterial  trunk  ;  in  A  also  the  outer  trunk  between 
the  third  and  fourth  arch  persists  as  the  ductus  Botalli  (rf) ;  in  A  and  B  the 
two  arches  of  the  fourth  series  are  seen  to  be  subequal,  to  cross  one  another 
just  outside  the  heart,  and  to  unite  behind  it  into  a  dorsal  aorta  (<7) ;  in  c  the 
left  fourth  arch  becomes  the  subclavian  artery,  and  takes.no  part  in  forming 
the  dorsal  aorta;  in  D  the  same  happens  to  the  fourth  arch  on  the ripht  side. 
In  all  the  primitive  fifth  arch  supplies  the  lungs;  but  in  mammals  tne  right 
kalf  disappears.  (After  Rathke.) 


respiration  is  largely  cutaneous  (see  page  326),  gives  off 
a  large  cutaneous  branch,  which  is  distributed  in 
the  skin. 


206    COM  PAR  ATI]/ E    ANATOMY   AND    PHYSIOLOGY. 

4.  The  aorta  proper  gives  off  large  branches,  such 
as  the  coeliaco-meseiiteric,  hepatic,  rcnals  to 

the  viscera  of  the  body  cavity,  and  two  large  hinder 
(iliac)  arteries  which  pass  to  the  hinder  limbs.  The 
final  termination  of  the  aorta  is  the  caudal  artery, 
and  the  size  and  extent  of  this  and  its  branches  are, 
of  course,  in  direct  relation  to  the  size  of  the  tail. 

As  the  arterial  vessels  get  smaller  and  smaller  the 
blood  from  them  flows  into  the  capillaries,  and  thence 
begins  to  make  its  way  back  to  the  heart  by  the 
veins. 

1.  The  blood  brought  back  by  the  hind-limb  enters 
the  pelvic  vein,  and  then  passes  either  by  the  iliac 
or  the  anterior  abdominal  into  the  vena  cava 
inferior ;  in  all  Vertebrates,  except  Mammals  and 
Birds,  the  blood  that  passes  along  the  iliac  veins  breaks 
up  again  into  a  system  of  smaller  veins  within  the 
substance  of  the  kidneys,  forming  in  them  a  renal 
portal*  system.  Again  collecting,  the  renals,  just  as 
in  mammals,  pass  into  the  inferior  vena  cava. 

3.  The  blood  from  the  intestinal  viscera  is  collected 
into  a  portal*  vein,  which  breaks  up  in  the  substance  of 
the  liver  into  a  portal  system  ;  this  hepatic  portal 
system  obtains  in  all  Vertebrates.  The  blood  brought 
from  the  liver  by  the  hepatic  veins  likewise  passes 
into  the  inferior  vena  cava. 

It  is  impossible  to  understand  the  arrangements  of 
the  collecting  portion  of  the  heart  and  of  the  superior 
veins  without  a  knowledge  of  what  obtains  in  all 
Vertebrates  at  an  early  period  of  development,  and 
in  Fishes  throughout  their  lives.  The  unpaired  vena 
cava  is  preceded  by  a  single  subintestinal  vein, 
which  collects  the  blood  from  the  yolk  sac,  and  this  by 

*  The  term  PORTAL  SYSTEM  owes  its  name  to  the  fact  that  in 
man  the  so-called  portal  vein  enters  the  substance  of  the  liver 
by  its  fissure  (or  PORTA,  as  the  older  Latin-speaking  anatomists 
called  it). 


Chap.  V.] 


VESSELS  OF  EMBRYO. 


207 


a  paired  system  of 
cardinal  veins, 
of  which  the  ante- 
rior pair  collect 
the  blood  from  the 
head,  and  the  pos- 
terior from  the 
body  generally 
(Fig.  90;  vc  and 
HC).  As  these  lie 
at  the  sides  of  the* 
body  the  blood  can 
only  pass  to  the 
heart  by  a  trans- 
verse vessel,  the 
diictus  Cuvieri 
(D).  These  two 
pairs  of  anterior 
and  posterior  car- 
dinal veins  are 
possessed  by  fishes, 
and  the  anterior 


par 
throughout 


remans 
the 


AA,  Abdominal  aorta  ;  RA. 
RA,  right  and  left  roots  of 
the  aorta,  connected  by 
the  collecting  vessels  (ss') 
with  the  branchial  ves- 
sels Ab;  cc',  carotids  ;  s6, 
subclavian  artery;  KL, 
gill  clefts ;  si,  sinus  ve- 
nosns ;  A,  atrium  ;  v,  ven- 
tricle ;  B,  bulbus  arterio- 
su* ;  VJH,  ompbalo-me?en- 
tcric veins:  Am.omphalo- 
mesenteric  arteries ;  ic 
ic,  common  iliac  arteries; 
EE.cxternal  iliac  arteries, 
A/?,alIantoic  arteries ;  Acd, 
caudal  artery ;  vc,  HC,  an- 
terior and  posterior  cardi- 
nal veins;  s//,  subclavian 
vein ;  D,  diictus  Cuvieri. 
(After  Wiedersheiiu.) 


Pig.  90.  -Diagram  to  explain  the  Arrange- 
ment of  the  Embryonic  vascular  System. 


208  CoMPARATfPM  ANATOMY  AND  PHYSIOLOGY. 

vertebrate  series  as  the  jugular  veins.  In  the  pen- 
tadactyle  vertebrates  the  chief  afferent  function  for 
the  hinder  part  of  the  body  is  undertaken  by  the  just 
mentioned  vena  cava  inferior,  the  posterior  cardinals 
becoming  the  pelvic  or  hypogastric  veins,  while 
the  Cuvieriaii  ducts  come  to  be  the  terminations  of  the 
jugular  veins,  and  so  lead  into  the  sinus  venosus 
whether  that  remains  distinct  from  the  atrium,  or,  as  in 
higher  forms,  becomes  a  portion  of  the  right  auricle. 

In  addition  to  the  important  pulmonary  veins 
which  open  into  the  left  auricle,  the  vessel  that  brings 
the  blood  from  the  fore-limb  (the  subclavian)  comes 
to  be  almost  as  important  as  the  jugulars. 

In  the  Mammalia  the  relations  of  the  superior  veins 
become  considerably  altered  ;  thus  the  left  jugular,  after 
having  entered  into  an  anastomosis  with  the  right, 
becomes  atrophied  in  various  Ungulates  and  Rodents  ; 
in  Man  and  the  other  Primates,  in  the  Carnivora,  and 
in  Cetacea,  the  left  jugular  ceases  to  have,  as  a  rule, 
any  rudiment  left  in  the  adult  condition.  Rudimentary 
left  ducts  of  Cuvier  have,  however,  been  occasionally 
observed  in  the  human  subject  (Quain,  Marshall). 

Although  there  is  a  general  body  of  truth  in  the 
statement  that  arterial  vessels  gradually  become 
smaller,  and  veins  larger,  as  the  one  leave  and  the 
other  approach  the  central  heart,  yet,  as  we  have 
already  seen  in  the  case  of  the  portal  circulation  in  the 
liver  and  kidney,  veins  do  break  up  into  smaller 
vessels  to  again  unite  into  a  larger  one  ;  when  this 
phenomenon  obtains  either  with  an  artery  or  with  a 
vein  in  any  other  part  of  the  body  than  the  organs 
just  mentioned,  such  a  plexus  of  vessels  is  ordinarily 
spoken  of  as  a  rete  mirabile. 

The  function  of  such  retia  is  not  far  to  seek,  the 
moment  we  remember  the  physical  law  that  the 
passage  of  a  fluid  is  slower  through  a  narrow  than  a 
wide  current ;  in  other  words,  the  exchange  of  gas  or 


Chap,  v.j  RETIA  MIRABILIA.  209 

the  escape  of  the  blood  corpuscles  (diapcedesis),  or,  in 
general  terms,  the  relation  of  the  moving  nutrient  and 
respiratory  currents  to  the  surrounding  cells  is  im- 
proved by  such  a  slowing.  This  will  explain  the 
presence  of  a  rete  mirabile  in  the  reduced  anterior  gill 
(pseudobranch)  of  many  fishes,  or  in  the  walls  of  their 
air  bladders,  for  by  this  means  the  exchange  of  oxygen 
is  the  more  successfully  effected  ;  a  similar  reason  may 
be  given  for  the  great  development  of  retia  in  the 
thoracic  and  costal  regions  of  the  Cetacea,  or  the  very 
general  distribution  of  small  plexuses  in  the  glomeruli 
of  the  kidney.  (See  page  258.) 

On  the  other  hand,  the  mere  mechanical  advantage 
of  the  slowing  of  the  blood  current  must  be  of  impor- 
tance in  forms  such  as  the  Ruminantia,  in  which  the 
head  is  often  for  a  long  period  at  a  lower  level  than 
that  of  the  carotids,  and  by  their  presence  the  dangers 
of  a  flow  of  blood  to  the  head  may  be  averted  ;  the  same 
kind  of  explanation  may,  no  doubt,  be  applied  to  the 
retia  in  the  course  of  the  abdominal  vessels,  the  pres- 
sure on  which  must  vary  greatly  with  the  extent  of 
distension  of  the  walls  of  the  intestinal  tract. 

It  is  more  difficult  to  explain  the  function  of  the 
retia  mirabilia  in  the  eyes  of  Fishes  and  Birds ;  or  in  the 
course  of  various  vessels  in  monotremes  and  edentates. 
In  the  latter  case  the  lowly  position  of  these  Mammals 
may,  perhaps,  be  of  significance. 

The  earlier  division  of  Vertebrates  into  hot-blooded 
and  cold-blooded  forms  disappeared  before  the  pro- 
gress of  morphological  discovery ;  in  addition  to  the 
evidence  that  we  now  have  as  to  the  relationship  of 
birds  to  reptiles,  we  have  further  to  support  us  the 
now  generally  recognised  difference  between  homoplasy 
and  homogeny  ;  in  other  words,  it  is  now  clear  to  us 
that,  with  given  structural  arrangements,  two  forms, 
not  closely  allied,  may,  under  similar  external  con- 
ditions, acquire  a  physiological  resemblance  ;  and  we 
0—16 


2io  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

now  see  that  Birds  and  Mammals  have  independently 
acquired  the  hob-blooded  condition.  In  connection 
with  this  it  is  important  to  observe  that  they  are  the 
only  Vertebrates  in  which  the  arterial  and  venous 
blood  do  not  commingle  at  some  point  in  the  circulatory 
system.  The  term  cold-blooded  is  so  far  inaccurate,  that 
the  blood  is  no  colder  than  the  surrounding  medium, 
whether  that  be  air  or  water  ;  and,  as  its  temperature 
varies,  it  is  better  to  use  the  term  "  poikilothermic." 


CHAPTER   VI 

ORGANS    OF    RESPIRATION. 

THE  essential  object  of  the  process  of  respiration  is 
the  taking  in  of  oxygen  either  from  the  atmospheric 
air,  which  may  be  said  to  be  oxygen  dissolved  in 
nitrogen,  or  from  water,  which  holds  in  solution  a 
larger  quantity  of  oxygen  than  of  nitrogen;  for, 
whereas  atmospheric  air  consists  of  twenty  volumes  of 
oxygen -to  eighty  of  nitrogen,  water  contains  about  33 
per  cent,  of  oxygen.  In  the  simplest  cases  (that  of  naked 
cells,  such  as  Amoeba)  the  oxygen  passes  directly 
into  the  protoplasmic  substance  of  the  cell,  and  the 
chief  product  of  the  oxydation  of  the  tissue,  carbonic 
acid,  passes  freely  out.  In  the  Infusoria  the  oxygen 
has  to  make  its  way  through  the  cuticle,  and  some,  no 
doubt,  enters  with  the  drops  of  water  that  are  taken 
in  by  the  mouth  ;  we  are  as  yet  altogether  ignorant  of 
the  conditions  under  which  animal  membranes  allow  of 
the  entrance  of  oxygen  and  the  exit  of  carbonic  acid. 
We  have  already  seen  that  the  cilia  are  of  use  in  driving 
food  particles  towards  the  mouth,  and  it  is  clear  that 
they  are  also  of  assistance  in  the  respiratory  process  by 
producing  currents  around  the  cell,  and  thereby  bring- 
ing fresh  supplies  of  oxygenated  water  to  its  surface. 


Chap,  vi.i  LOWER  METAZOA.  211 

The  cilia  of  the  flagellated  chambers  of  the 
Sponge  (Fig.  53)  have  clearly  the  same  function 
among  the  Porifera,  and  the  currents,  which  we  have 
already  learnt  to  be  food-bearing  currents,  are  the 
means  also  by  which  oxygen  is  brought  to  the  cells 
that  line  the  surface  of  the  canals,  and,  while  they 
bring  oxygen,  they  carry  away  carbonic  acid.  Similarly, 
among  the  Ccelenterata,*  the  necessary  oxygen 
enters  in  the  same  way,  and  passes  by  the  same 
passages  as  the  food-currents.  Among  the  lower 
worms,  such  as  the  Turfoellaria,  oxygen  is,  no 
doubt,  obtained  in  the  same  way,  but  here  in  all 
probability  the  soft  ciliated  integument  also  affords  a 
means  by  which  oxygen  may  be  carried  to  the  cells. 

It  would  be  difficult  to  point  to  any  part  of  the 
body  of  a  Nematoid  worm  which  could  be  reason- 
ably supposed  to  have  a  respiratory  function  ;  on  the 
other  hand,  such  experiments  as  those  of  Oerley,  who 
placed  a  number  of  specimens  of  Anguillula  aceti  (the 
vinegar  paste-worm)  in  a  small  vessel  and  covered 
them  with  a  layer  of  oil  an  inch  thick,  and  found  that 
after  two  months  the  greater  number  were  still  alive, 
prove  that,  in  these  worms  at  any  rate,  there  is  but  a 
very  feeble  demand  for  fresh  supplies  of  oxygen. 

Where  a  definite  blood-vascular  system  is  de- 
veloped, the  consideration  of  respiratory  problems  is 
rendered  easier ;  for,  just  as  the  currents  of  water  in  a 
sponge  carry  in  oxygen  as  well  as  food,  so  does  the 
blood  of  the  higher  Ifletazoa  serve  as  an  oxygen 
carrier  as  well  as  a  store  of  food  material  for  the 
different  cells  of  the  body.  The  oxygenating  office  is 
again  rendered  still  more  effectual  when  the  blood 

*  It  has  been  recently  pointed  out  by  Lankester  that  the  so- 
called  SUBGENITAL  PITS  of  Aurelia  have  no  connection  at  all  with 
the  geni-fcal  glands.  They  are  spacious  cavities,  opening  to  the 
exterior  by  comparatively  small  pores,  and  they  serve,  he  thinks, 
as  receptacles  for  respiratory  water.  They  are  four  in  number, 
and  are  set  close  to  the  mouth,  one  in  each  quarter  of  the  disc. 


212   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

is  impregnated  with  haemoglobin  (see  Power's 
"  Human  Physiology,"),  which  has  a  remarkable 
affinity  for  oxygen.  This  haemoglobin  is  either  found  in 
special  corpuscles,  as  in  the  Yertebrata,  some  Annelids 
(Glycera,  Capitella),  some  Gephyreans  (Phoronis, 
Thalassema,  Hamingia),  some  Lamellibranchs  (Solen, 
Area),  or  it  is  diffused  in  the  liquid  of  the  blood,  as  in 
most,  though  not  all,  Polychsetous  and  Oligochsetous 
Annelids  and  Hirudinea,  some  Nemertines,  some 
Crustacea,  the  mollusc  Planorbis,  and  the  larva  of  the 
insect  Chironomus.*  It  has  been  found  in  a  special 
system  of  vessels  in  the  Crustacean  Lernanthropus  and 
Clavella,  and  in  the  corpuscles  of  the  water  vascular 
system  of  Ophiactis  virens.  In  various  forms  it  is 
found  diffused  in  muscular  tissue.  Its  most  remark- 
able position,  however,  is  in  the  nerve  tissue  ;  this  has 
been  observed  in  the  sea-mouse  (Aphrodite  aculeata) 
and  in  a  number  of  Nemertinea ;  in  the  latter  it  is 
absent  from  the  nerve-tissue  of  those  forms  in  which 
the  blood  is  impregnated  with  it. 

f)  Among  the  higher  worms  in  which  no  special 
respiratory  apparatus  is  developed,  it  will  probably  be 
frequently  observed,  as  has  already  been  the  case  with 
the  leech,  that  the  epithelial  covering  of  the  body  is 
interpenetrated  by  capillaries  (Fig.  91);  "the  true 
respiratory  organ  of  the  leech  is  clearly  this  vascular 
epiderm,  and  amongst  respiratory  organs  it  stands 
alone  in  the  nearness  with  which  the  absorbent  blood- 
vessels succeed  in  bringing  themselves  through  all 
structural  obstacles  into  direct  contact  with  the 
oxygenating  medium  "  (Lankester). 

When  specialised  respiratory  organs  are  developed 
they  may  arise  in  various  ways,  either  as  in-pushings 
of  the  surface,  which  may  become  slits  or  tubes,  or  as 

*  The  larger  number  of  these  cases  have  been  observed  by  Ray 
Lankester.  See  the  Proceedings  of  the  Royal  Society,  1873,  No. 
140. 


Chap,  vi.]  GILLS.  213 

out-pushings  of  the  body  (external  gills) ;  or  water  may 
pass  through  or  into  the  intestine  and  be  pumped  out 
of  it  again,  or  the  walls  of  the  intestine  may  give  rise 
to  cavities  into  which  air  is  received ;  or,  lastly,  the 
body  wall  may  give  rise  to  an  air  chamber  by  folding 
over  and  becoming  attached,  for  a  more  or  less  con- 
siderable extent,  to  the  surface  of  the  body,  as  is  the 
case  in  the  snail. 

In  a  general  way  we  apply  the  term  gill  to  an 

eji  ,cu 


Fig.  91. — From  a  Transverse  Section  of  a  Leech.;  to  show  cu,  cuticle  ; 
v,  infra-epithelial  blood-vessel ;  ep,  epithelial  cells.  (After  Ray 
Lankester. ) 

organ  which  is  fitted  to  take  up  the  oxygen  dissolved 
in  water,  and  lung:  to  that  which  breathes  the  oxygen 
of  atmospheric  air ;  but  these  terms  can  only  be  used 
in  a  very  general  sense. 

The  simplest  form  of  in-pushing  is  seen  in  the 
Nemertinea,  where  the  so-called  side-organs  or 
ciliated  furrows  are  in  Carinella  annulata  (Fig.  92  ;  A) 
mere  pits  ;  this  pit  in  C.  inexpectata  forms  a  more 
complicated  groove,  which  leads  into  a  ciliated  duct 
(Fig.  92  ;  B),  which  ends  blindly  among  the  cells  of  the 
brain  ;  in  both  these  species  the  nervous  centres  are 
quite  close  to  the  surface  (page  398),  and  the  tissue  is, 
as  we  have  just  learnt,  impregnated  with  haemoglobin. 
In  Polia,  where  the  brain  is  more  deeply  placed, 
the  ciliated  duct  is  longer  (Fig.  92;  c).  In  the 


214  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Sehizonemertini  (such  as  Cerebratulus)  the  transverse 
furrow  of  the  Palseonemertine  gives  place  to  a  deep 
longitudinal  slit,  which  penetrates  the  muscular  tissue 


Fi&.  92.— Diagrams  of  the  E espiratory  Ciliated  Ducts  of  (A)  Carinella 
annulata;  (B)  C.  inexpectata ;  (c)  Polio  curia;  (D)  Certbratulus. 
(After  Hubrecht.) 

of  the  head,  and  is  continued  below  into  the  ciliated 
canal  which  penetrates  the  lobe  of  the  brain  (Fig. 
92;  D).  The  respiratory  function  of  these  slits  has 
been  pointed  out  by  Hubrecht,  who  has  made  experi- 
ments in  demonstration  of  his  hypothesis,  and  who 


Chap.  VI.] 


TRACHEJE. 


215 


has  directed  attention  to  the  deeper  slits  and  larger 
amount  of  haemoglobin  in  the  Schizonemertini,  as 
being  correlated  with  their  habit  of  dwelling  in  mud 
and  other  places  in  which  the  supply  of  oxygen  is 
small;  the  ciliated  cells  of  the  groove  clearly  serve  to 
drive  in  currents  of  sea-water. 

In  c  and  D  (Fig.  92)  it  will  be  observed  that  the 
lower  part  of  the  ganglionic  mass  is  shaded  more  lightly 
than  the  rest ;  the  cells  that  form  this  portion  are 
derived  from  the  ossophagus,  from  the  walls  of 
which,  however,  they  subsequently  become  separated ; 
and  we  observe,  therefore,  that  an  out-pushing  from 
the  gullet  goes  to 
meet  the  epiblastic 
in- pushing  from  the 
surface. 

An  essentially 
similar  phenomenon 
is  to  be  observed 
in  the  Entero- 
pneusti  (Balano- 
glossus)  and  in  the 
Choi-data.  (See 
page  231.) 

In  the  trache- 
ate  Artliropoda 
we  have  examples  of 
in-pushings  of  the 
surface  adapted  for 

the  entrance  not  of  water,  but  of  air  ;  they  are  seen  at 
their  simplest  in  Peripatus,  where  they  are 
distributed  over  the  whole  of  the  body.  The  tracheal 
orifice  leads  into  a  pit,  which  traverses  the  dermis, 
and  widens  out  at  its  inner  end ;  from  this  the 
tracheae  arise  as  minute  tubes,  which  rarely  branch, 
and  only  have  a  faint  indication  of  the  presence  of  the 
spiral  fibre,  which,  in  higher  Tracheates,  gives  so 


Fig.  93.— Tracheae  of  Insect,  showing  the 
Spiral  Fibre. 


2i6   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

characteristic  an  appearance  to  these  tubes  (Fig.  93). 
These  tracheae  are  distributed  to  all  parts  of  the 
body,  running  alongside  the  nerves  and  entering  with 
them  into  the  central  nervous  system ;  they  are 
particularly  well  developed  in  the  head. 

In  the  higher  forms  the  tracheal  orifices,  which  are 
definitely  known  as  stigmata,  are  not  distributed  over 
the  whole  body ;  they  would  appear  to  have  been  primi- 
tively arranged  by  pairs  in  each  metamere,  and  in  some 
Myriopods  that  condition  is  essentially  retained.  As 
the  stigmata  diminish  in  number,  the  tracheae  tend  to 
branch,  and  in  some  Myriopods  (such  as  lulus) 
they  unite  with  one  another  and  form  tracheal 
anastomoses  of  much  the  same  kind  as  those  found 
among  Hexapod  insects  ;  in  the  higher  members  of 
the  last-mentioned  group  the  stigmata  are  reduced  to 
two  or  three  pairs,  but,  thanks  to  the  anastomosing 
branches,  all  the  parts  of  the  body  are  still  well  supplied 
with  air  vessels,  the  thinnest  and  finest  of  which 
are  without  the  spiral  fibre.  Just  as  in  Birds 
(see  page  239),  we  find  that  in  flying  insects  the  air 
passages  or  tracheal  tubes  often  pass  into  swellings  or 
enlargements,  the  so-called  tracheal  vesicles  ;  these 
may  be  small  and  numerous  as  in  some  Coleoptera,  or 
larger  and  less  numerous  as  in  butterflies  and  bees, 
or  reduced  to  two,  which  are,  as  in  the  fly,  of  enor- 
mous size,  and  abdominal  in  position.  The  stigma 
or  spiracle,  by  which  the  air  enters,  varies  consider- 
ably in  form  and  structure,  and  the  same  is  true  of 
the  apparatus  by  which  it  can  be  closed.  Little  is  known 
with  certainty  as  to  the  mechanism  of  the  respira- 
tory movements  of  insects,  but  the  high  temperature 
of  the  bee-hive,  and  the  activity  of  such  restless 
creatures  as  the  house-fly,  speak  clearly  enough  of  a 
large  amount  of  oxydation,  and  of  waste  of  tissue  ; 
in  direct  relation  to  the  activity  of  the  cellular  organ- 
ism is  the  amount  of  oxygen  which  it  requires.  It 


chap,  vi.]         RESPIRATION  OF  INSECTS.  217 

has  been  observed  by  Lowne  that  the  blowfly  makes 
vigorous  movements  with  its  legs  from  sixteen  to 
thirty  times  a  minute,  and  he  judges  that  these  are 
respiratory  movements  from  the  consideration  that, 
as  the  valves  of  the  anterior  spiracles  are  closed  for  a 
short  time,  the  air  must  be  necessarily  driven  through 
the  small  tubes  \  these  movements  are  accompanied 
by  a  contraction  and  a  dilatation  of  the  abdomen. 
If  placed  under  an  exhausted  receiver  of  an  air-pump 
and  removed  before  death  ensues,  the  refilling  of  the 
tracheal  system  is  observed  to  be  accompanied  by 
violent  movements  of  the  legs  and  wings,  so  that 
here,  as  in  the  crayfish  (see  page  225)  and  elsewhere, 
the  activity  of  the  locomotor  organs  is  seen  to  be  in 
direct  relation  with  an  increased  supply  of  oxygen. 
In  the  larvae  of  some  Insects  the  tracheal  system  is 
completely  closed,  or,  in  other  words,  there  are 
tracheal  tubes  but  no  stigmata;  in  these  cases  the 
tubes  are  either  placed  not  far  from  the  surface  of 
the  body,  when  the  respiration  may  be  said  to  be 
vague,  or  the  tracheae  form  out-growths  (the  so- 
called  tracheal  gills)  which  project  into  the  water 
(e.g.  Culicidae),  and  offer  the  exact  converse  of  what  is 
the  essential  point  in  tracheal  respiration ;  in  it  the 
air  comes  to  the  organs  by  internal  tubes,  in  such 
larvae  tubes  go  to  the  water  in  order  to  obtain  the 
oxygen. 

*  We  may  appropriately  pass  from  these  external 
^tracheal  tubes  to  the  form  of  respiratory  organ  in 
which  there  is  an  out-pushing  of  the  body  wall  to 
form  a  so-called  external  gill.  Physiologically  this 
condition  is  preceded  by  what  obtains  in  the  Starfish, 
where  the  respiration  is  said  to  be  vague ;  from 
among  the  spaces  left  between  the  ossicles  of  the 
dorsal  surface  (Fig.  23)  there  project  thin  membranous 
out-pushings  of  the  lining  of  the  body  cavity,  and 
the  fluid  within  is  thus  only  separated  from  the 


218  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

oxygenated  sea-water  by  a  thin  wall  of  membrane 
through  which  oxygen  passes  inwards,  and  carbonic 
acid  outwards.  In  the  Ophinroidea,  where  the 
calcareous  skeleton  consists  of  continuous  plates,  a 
slit  is  to  be  seen  on  either  side  of  the  insertion  of  each 
arm  into  the  disc ;  these  so-called  genital  slits  or 
bursal  clefts,  as  they  are  more  appropriately 
named  by  Ludwig,  lead  into  membranous  sacs  (bursse) 
into  which  sea-water  is  sucked,  and  from  which  it 
is  again  driven  out ;  the  thin  walls  of  these  sacs 
project  into  the  body  cavity,  and  here  again  the 
oxygenated  sea-water  is  separated  from  the  contents 
of  the  crelom  by  nothing  more  than  a  thin  wall.  In 
the  Ecltinoidea,  where  the  test  is,  again,  con- 
tinuous, the  membranous  sacs  are  either  internal  as 
in  Cidaris,  where  they  lie  at  the  apex  of  the  lantern 
of  Aristotle,  or  they  form  five  pairs  of  sacculated 
projections  which  protrude  from  the  five  pairs  of 
slits  in  the  margin  of  the  mouth.  In  some  Echinoids 
the  tube  feet,  or  ambulacral  suckers,  take  on  a  respi- 
ratory function.  In  the  Holotlmroidea,  the  respi- 
ratory organs,  when  specially  developed,  are  connected 
with  the  intestinal  tract.  (See  page  229.) 

A  simple  condition  of  external  gill  obtains  in 
some  of  the  Polyclisetous  Annelids,  where, 
however,  they  are  by  no  means  always  developed.  In 
Nereis,  for  example,  they  are  simple,  short  filaments ; 
in  Cirratulus  the  filaments  are  simple,  but  are  produced 
to  an  extraordinary  length ;  in  others,  as  the  lug-worm 
(Arenicola),  they  are  comparatively  short  but  are 
elaborately  branched  (Fig.  94) ;  in  the  tube-dwellers 
they  are  confined  to  the  anterior  end,  where  they  form 
a  pair  of  "  respiratory  plumes."  In  all  cases  these 
gills  consist  essentially  of  a  thin  wall,  the  epithelium 
of  which  is  ciliated,  and  within  which  are  blood- 
vessels more  or  less  well  developed.  In  the  plumes  of 
the  Tufoicolse  supporting  cartilages  are  not  rarely 


Chap.  VI.] 


GILLS  OF  MOLLUSC  A. 


219 


developed.  The  mechanism  of  these  gills  is  simple 
enough  ;  projecting  into  the  water  they  are  moved 
about  in  it,  and  minor  currents  are  produced  around 
them  by  the  action  of  the  cilia  which  cover  their 
surface. 

A  very  simple  type  of  gill  is  found  in  some  of  the 
Lamellibranchiata,  where  the  Ctenidiiim  (see 
page  79)  consists  essentially  of  two  rows  of  hollow 
filamentar  processes  of  the  body  wall  extended  along 
either  side  of  the  foot,  and  ciliated  externally ;  these 
are  the  gill  filaments;  each  filament,  at  its  free 
end,  bends  upwards  in  such  a  way  as  to  leave  a  space 
between  the  ascending  and  descending  portions ;  the 
free  end  of  the  ascending  portion  of  the  outer  filament 
may  either  be  free,  as  in  the  sea-mussel  (Mytilus),  or 
become  at- 
tached to 
the  mantle 
as  in  the 
fresh  -  water 
mussel  (An- 
odon)  ;  it  is 
plain,  then, 
that  its  as- 
cending lies 
externally 
to  its  de- 
scending 
portion ;  the 

inner  gill  filament  has  its  ascending  portion  internal  to 
the  descending,  and  so  it  results  that  in  transverse  sec- 
tion the  two  gill  filaments  have  the  form  of  a  W,  the 
median  upper  point  of  which  represents  the  point 
where  the  two  filaments  are  inserted  (or  arise  from) 
the  body  wall.  The  ciliated  epithelial  cells  are  often 
particularly  well  developed  at  certain  spots ;  the  cilia 
of  these  "  ciliated  junctions  "  interlock  with  those 


•n/ 


Fig.  94.— Transverse  Section  of  Arenicola. 

D.  Dorsal ;  v,  ventral  side ;  n,  ganglionic  chain  ;  i,  intes- 
tine ;  br,  gills;  v,  ventral  vessel;  d,  dorsal;  t/,  visceral 
vessel ;  p,  vessel  around  intestine ;  a,  b,  vessels  of  gills. 
(After  Uegenbaur.) 


22o  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


of    a    corresponding    junction    on     a     neighbouring 
filament   (Fig.   95  ;  A)  ;  and  in  this  way  the   several 

filaments  are  connected 
together.  But  the  concre- 
scence of  the  constituent 
parts  of  a  gill  does  not  end 
with  the  union  of  the 
neighbouring  filaments,  the 
ascending  and  descending 
portions  likewise  become 
connected  with  one  another 
by  iiiterlame liar  June- 


IF!!  „.£ 


.Jl-     .J 


"V  k  i 

1 


Fig.  95.— A,  Part  of  three  gill  filaments  of  Area,  showing  the  large 
ciliated  junctions;  B,  transverse  section  of  a  portion  of  an  outer 
gill  plate,  with  solid  interlamellar  junctions  and  large  vertical 
vessels  of  Anodon ;  c,  a  more  highly  magnified  view  of  B.  (After 
K.  HolmanPeck.) 

tions  (Fig.  95  ;  B  and  c),  and  the  bridges  of  union  that 
were  primitively  ciliated  become  fibrous  and  solid,  till 
at  last  we  reach  the  continuous  plate-like  arrangements, 
which  are  channelled  by  spaces,  or  by  definite  vessels 
which  lie  outside  the  primitive  blood-carrying  hollow 


Chap,  vi.]  GILLS  OF  MOLLUSC  A,  221 

gill  filaments,  and  which  are  now  the  means  of  passage 
for  the  circulatory  medium.  With  this  considerable 
alteration  in  structure  there  would  seem  to  have  come 
some  change  of  function,  and  Peck  is  probably  right 
in  believing  that  the  ciliated  gill  plates  of  the  Lamelli- 
branchs,  by  causing  a  constant  current  of  water  to  pass 
over  the  animal,  are  as  important  aids  to  their  nutrient 
as  to  their  respiratory  processes.  The  spaces  between 
the  gill  filaments  are  sometimes  used  as  incubatory 
pouches,  in  which  the  Olochidia,  or  young  of  the 
fresh -water  mussel,  lie  during  the  early  days  of  their 
development. 

The  branchial  outgrowths  (ctenidia)  of  the 
Cephalophora  are,  on  the  whole,  simpler  in  structure 
than  in  the  more  highly  developed  members  of  the 
Lamellibranchiate  group  ;  in  most  of  the  lowest  forms 
they  are  arranged  in  a  circle,  as  in  Neomenia,  where 
there  is  an  oblong  circlet  of  thirty  filiform  tubules,  the 
Chiton,  or  the  limpet ;  in  Haliotis  and  others  they 
are  distinctly  bilateral,  and  in  the  Anisobranchiata 
the  left  gill  is,  owing  to  the  torsion  of  the  body,  much 
smaller  than  the  right;  in  others,  as  the  Hetero- 
]>ocla,  the  left  gill  disappears  altogether,  and  in  some 
the  right  gill  has  only  one  lamella  developed  ;  this  is 
the  so-called  semi-pi iinate  gill*  In  other  Oastro- 
pods,  such  as  Lymnseus,  or  Helix,  the  gills  disappear 
altogether,  and  a  so-called  lung  is  developed.  (See 
page  228.)  In  a  number  of  naked  Pteropoda  re- 
spiration becomes  vague  by  a  process  of  adaptative 
degeneration ;  or,  in  other  words,  the  whole  surface  of 
the  body  takes  on  the  function  of  the  lost  gills.  Among 
the  Cephalopoda  the  Tetrabranchiata  (Nautilus) 
have  two  pairs,  and  the  Dibranchiata  (Octopus,  Loligo) 
one  pair  of  well-developed  gills. 

In  the  great  majority  of  cases  the  gills  of  the 
Mollusca  are  covered  in  by  the  mantle,  and  come, 
therefore,  to  lie  in  a  branchial  chamber;  into 


222   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

this  chamber  the  air  is,  under  the  simplest  conditions, 
drawn  in  by  the  action  of  the  cilia  which  cover  the 
surface  of  the  gills  or  gill  plates ;  in  the  Anodon,  for 
example,  the  currents  of  water  enter  into  the  lower 
part  of  the  chamber,  which  in  the  hinder  region  of  the 
body  is  separated  from  the  upper  by  the  union  of  the 
gill  plates  of  either  side  along  the  middle  line  ;  the 
water  that  enters  by  this  lower  inhalent  passage  passes 
out  by  the  upper  or  exhalent  one.  In  a  number  of 
Lamellibranchs  the  mantle  which  bounds  these  orifices 
is  produced  into  a  more  or  less  long  siphon ;  these 
siphons  are  best  developed  in  forms  that  burrow  in 
the  sand,  and  which  have  the  siphons  directed  upwards. 
A  similar  kind  of  gill  chamber  is  formed  in  many 
Gastropods  by  the  folding  over  of  the  mantle, 
and  in  a  number  of  flesh-eating  forms  a  pair  of 
siphons  are  also  developed.  The  absence  of  the 
mantle-fold  in  such  forms  as  the  JVudibranehs 
leads  us,  physiologically,  to  the  vague  respiration  of 
the  gynmosomatous  Pteropoda,  where  the  gills  have 
become  atrophied. 

The  most  characteristic  organ  of  the  true 
Cephalopoda  is  the  so-called  funnel,  which  is  a 
modification  of  part  of  the  foot ;  in  these  highly 
developed  molluscs  we  have  again  an  example  of  the 
relation  of  the  respiratory  to  the  locomotor  activity 
of  the  animal.  When  the  muscles  in  the  walls  of  the 
investing  mantle  relax  they  allow  water  to  enter  into 
the  gill  chamber  on  either  side  of  this  funnel ;  when 
they  contract  they  not  only  press  on  the  water  in  the 
cavity,  but  also  close  the  orifices  by  which  it  entered ; 
the  only  passage,  then,  by  which  the  compressed  fluid 
can  escape  to  the  exterior  is  by  way  of  the  funnel,  the 
walls  of  which,  by  contracting,  aid  in  the  expulsion  of 
the  water ;  and  the  final  result  of  this  expulsion  is, 
that,  unless  the  Cephalopod  is  resting  on  the  ground 
it  is  driven  backwards  ;  the  more  of  ben  then,  water  is 


VI.] 


GILLS  OF  CRUSTACEA. 


223 


baken  in  and  driven  out,  the  more  often  is  the  animal 
mechanically  helped  on  its  course. 

In  no  group  are  gills  better  or  more  characteristi- 
3ally  developed  than  in  Crustacea,  and  in  none  do 
we  find  better  evidence  of  the  association  of  locomotor 
with  respiratory  activity.  Among  the  lowest  repre- 
sentatives of  the  group  (the  BrancSiiopoda)  a 
number  of  the  appen- 
dages are  nothing  more 
than  broadened  thin 
plates  within  which 
the  blood  circulates, 
and  outside  of  which  is 
the  oxygenated  water  in 
which  they  are  bathed 
(Fig.  96). 

In  Squilla  and  its 
allies  branched  tufts  of 
gill  filaments  are  at- 
tached to  the  abdomi- 
nal feet. 

In  the  Decapoda, 
such  as  the  crayfish 
or  the  lobster,  the 

gills  are  outgrowths  of  the  sides  of  the  body  wall,  but 
fcheir  relation  to  the  locomotor  function  is  still  well 
marked ;  in  this  group  the  gills  are  placed  in  a  gill 
chamber,  which,  as  it  is  formed  by  lateral  folds  of  the 
dorsal  integument,  reminds  us,  so  far,  of  the  simpler 
arrangements  of  the  Branchiopod  (Fig.  96 ;  d) ; 
these  gills  are  set  in  three  sets,  the  lowest  of  which 
are  (in  the  nomenclature  of  Huxley)  podobranehs, 
for  they  are  attached  to  the  basal  joints  of  the  appen- 
dages (in  the  crayfish  from  the  second  maxilliped  to 
the  penultimate  thoracic  appendage) ;  the  next  set, 
which  are  arranged  in  two  rows,  are  called  the 
arthrobranchs,  from  the  fact  that  they  are  attached 


Fig.  96. — Transverse  Section  of 
a  Branchiopod,  showing  the 
leaf-like  (phyllopod)  gills 
(br'),  which  are  appendages 
of  the  body. 

c,  Heart ;  i,  intestine  ;  n,  ventral  nerve- 
cord  ;  d,  fold  of  the  iutegument. 
(After  Grube.) 


224    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

to  the  membranous  piece  which  connects  the  basal 
joints  of  the  appendages  with  the  walls  of  the  thorax: 
the  third  and  uppermost  set  consists  of  pleuro- 
branctis,  so-called  from  their  attachment  to  the  sides 
of  the  thorax.  Among  different  members  of  the 
group  we  find  a  difference  in  the  number  of  these 
gills,  and  here,  as  elsewhere,  in  scientific  investiga- 
tions, much  time  is  saved,  and  intellectual  operations 
considerably  aided,  by  the  use  of  formulae.  The  fol- 
lowing table,  taken  from  Huxley,  may  be  regarded  as 
a  type  which  is  to  be  followed  out  when  making  the 
dissection  of  any  one  of  the  higher  Crustacea. 


HYPOTHETICALLY  COMPLETE  BRANCHIAL  FORMULA. 


Somites  and 

lagVIL* 

VIII. 

IX. 

X. 

XI. 

XII. 

XIII. 

XIV. 


Arthol>ranchs. 
Anterior.       Posterior. 

1 
1 


Pleurohranchg. 

=  4 

=  4 

=  4 

=  4 

=  4 

=  4 
4 


=       32 


All  these  gills  have  essentially  the  same  structure ; 
they  consist  of  an  elongated  stem,  within  which 
run  two  distinct  canals,  into  one  of  which  the  blood 
passes  from  the  body,  and  by  the  other  of  which  it 
returns  on  its  way  to  the  heart ;  connected  with  these 
canals  are  a  number  of  comparatively  short  hollow  fila- 
ments with  thin  walls ;  the  blood,  therefore,  on  pass- 
ing into  them,  is  separated  only  by  a  thin  membrane 
from  the  oxygenated  water  that  is  passing  through  the 
gill  chamber,  and  an  exchange  of  carbonic  acid  and 


*  There  are  many  reasons  for  beginning  to  count  tl 
one  farther  back,  and  to  call  vi.  what  Prof.  Huxley  call 


to  count  the  segments 
"a  vii. 


Chap,  vi.]  GILLS  OF  CRUSTACEA.  225 

oxygen  is,  consequently,  easily  effected.  The  podo- 
branchs  and  pleurobranchs  are  more  elaborately  con- 
stituted than  the  simpler  arthrobranchs. 

When  we  remember  the  well-known  fact  that  the 
Crustacea  are  altogether  devoid  of  cilia,  we  find  it  at 
first  difficult  to  understand  how  water  is  driven 
through  the  gill  chamber  ;  we  have  only,  however,  to 
make  the  experiment  to  see.  that  a  current  of  water  does 
constantly  enter  at  its  hinder  and  pass  out  at  the 
anterior  end.  The  apparatus  by  which  this  current  is 
produced  is,  again,  a  modification  of  one  of  the  appen- 
dages for  the  exopodite  and  epipodite  of  the  second 
maxilla  (page  1 23)  of  either  side  is  converted  into  a  scoop- 
shaped  plate,  the  cavity  of  which  is  directed  forwards, 
and  which  itself  fits  into  the  anterior  orifice  of  the 
gill  chamber ;  this  so-called  scaphognatliite  moves 
backwards  and  forwards  about  200  times  a  minute, 
and  with  each  backward  and  forward  movement  it 
scoops  out  water  at  the  anterior,  and  causes  a  fresh 
supply  of  water  to  enter  at  the  hinder  end  of  the 
animal ;  moreover,  the  quicker  the  animal  moves, 
the  quicker  the  action  of  the  scaphognathite,  and,  in 
consequence,  the  larger  the  inflow  of  oxygenated  water. 
Within  the  gill  chamber  the  waving  plumes  of  the 
gills  aid  in  the  movement  of  the  water,  and  the  at- 
tachment of  the  podobranchs  to  the  ambulatory 
(thoracic)  and  hinder  mandibular  appendages  affords  a 
certainty  that  the  more  these  appendages  work  the 
greater  will  be  the  supply  of  oxygen  that  they  will 
receive. 

A  few  Crustacea  are  modified  to  breathe  air.  (See 
below. ) 

Among  the  Arachnida,  processes  of  the  body 
projecting  into  the  water  are  found  in  the  only  mem- 
bers of  the  group  that  are  inhabitants  of  the  sea; 
these  are  the  King-crabs,  represented  to-day  by  Li- 
mulus.  In  these  forms  there  is  no  protecting  gill 
p— 16 


226  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

chamber,  and  the  gills  project  freely  into  the  water,  as 
in  the  lower  Crustacea ;  they  are  attached  to  the  five 
pairs  of  abdominal  feet,  and  are  broad,  flattened,  and 
provided  with  a  number  of  secondary  plates.  These 
gills  are  essentially  formed  by  a  hard  supporting  axis, 
on  which  are  placed  some  hundred  and  fifty  flattened 
lamellae  (Fig.  97) ;  these  lamellae  are  exposed  to  the 
surrounding  water,  and  as  they  are  hollow,  and  their 
cavities  contain  blood,  we  have  only  further  to  know 
that  their  walls  are  delicate  to  understand  how  it  is 
that  these  "gill-books"  are  respiratory  organs. 
Connected  with  these  gills  are  the  so-called  "  lung- 
books  "  of  the  scorpion,  which  are  adapted  for  aerial 
respiration,  and  the  exact  characters  of  which  have 
been  very  carefully  and  ingeniously  elucidated  by 
Lankester.  In  that  Arachnid  the  ninth  to  the  twelfth 
segments  bear  appendages  which  are  respiratory  in 
function,  the  appendages  of  the  eighth  segment,  or  the 
first  which  is  branchial  in  Limulus,  being  more  espe- 
cially modified  as  the  so-called  "  pectines."  These 
respiratory  appendages,  or  "  lung-books,"  are,  like  the 
"  gill-books "  of  the  king-crab,  formed  essentially  by 
a  hard  supporting  axis,  and  a  number  of  lamella  set 
on  that  axis  (Fig.  97,  B  ;  and  in  greater  detail  c)  ;  but 
they  differ,  at  first  sight,  altogether  from  those  of  the 
king-crab  in  being  no  longer  exposed  freely,  but 
placed  in  recesses,  which  open  to  the  exterior  by  a 
narrow  slit.  This  slit  gives  entrance  not  to  blood  but 
to  air,  and,  as  it  communicates  with  the  cavities  in  the 
lamellse  of  the  lung-book,  we  expect  to  find  that  these 
do  not,  as  in  Limulus,  any  longer  serve  as  blood 
passages  ;  the  blood,  indeed,  is  now  found  in  the  sac 
which  invests  the  lung-book.  Great  as  are  the  struc- 
tural changes,  the  ultimate  physiological  arrangement 
is  the  same  as  before  ;  in  other  words,  the  lung-books 
no  less  than  the  gill-books  are  respiratory  organs,  but, 
instead  of  carrying  the  blood  to  the  oxygen,  they  carry 


Chap.  VI.] 


ARACHNIDA. 


227 


the  oxygen  to  the  blood.     Among  the  Arachnida,  thb 
remarkable  series  of  changes  so  far  followed   does  not 


Fig.  97. — A.  View  of  the  lower  Margin  of  the  right  lamelliferous  Appen- 
dages of  the  eleventh  segment  of  Limulus  polyphemus. 

A.  I,  Proximal  lamella  ;  lx,  one  hundred  and  fiftieth  lamella  ;  ex,  external  lappet  of 

the  bifld  distal  prolongation  of  the  appendages. 
B.   A  similar  view  of  the  corresponding  appendage  of  Buthus  fcocTii. 

lx,  One  hundred  and  thirtieth  lamella  ; 
c.   A.  semi-diagramatic  view  of  one  of  the  respiratory  Appendages  of  a 

Scorpion,  to  show 

I,  The  bases  of  the  lamellce  exposed  by  the  removal  of  the  integument  of 
the  axis,  the  remnants  of  which  are  seen  at  TO  :  oc,  the  projecting  portion  of 
axis ;  I,  proximal  lamella.  (After  E.  Ray  Laukester.) 

end  with  the  scorpion  ;  some,  like  Mygale,  have  but 
two  pairs  of  lung-books,  and  other  spiders  have  but 


228  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

one.  In  others,  like  the  mites,  there  are  no  specia- 
lised respiratory  organs,  but  a  "  vague  respiration  ; " 
and,  lastly,  others,  such  as  the  pseudo-scorpions 
(Chelifer),  have  replaced  the  lung  -  books  by  true 
tracheal  tubes. 

A  number  of  Mollusca  have,  like  the  common 
snail,  replaced  an  aquatic  method  of  respiration  by 
one  that  is  aerial ;  this  is  effected  by  the  large  distri- 
bution of  blood-vessels  to  a  part  of  the  mantle,  which 
becomes  so  attached  to  the  sides  of  the  body  as  to  leave 
only  a  comparatively  small  orifice  by  which  air  can 
enter ;  the  modified  mantle  chamber  is  called  the 
44  lung1.99  This  arrangement  is  not  separated  by  any 
wide  gulf  from  that  which  is  found  in  the  branchiate 
Gastropoda,  for  some  of  these  have  the  walls  of  the 
mantle  cavity  more  or  less  well  provided  with  a  lung  ; 
and  others,  like  the  marsh-snail  (Paludina),  have  both 
gill  and  lung.  On  the  other  hand,  the  water-snail 
(Lymnseus)  has  no  gill  at  all,  yet  constantly  lives  in 
water,  and  uses  its  air  receptacle,  as  do  some  fishes 
(page  232),  as  a  hydrostatic  organ. 

In  some  cases,  as  Semper  has  pointed  out,  certain 
Mollusca  may  be  truly  spoken  of  as  amphibious ; 
Ampullaria,  an  ally  of  Paludina,  has  been  observed 
by  him  to  use  its  gills  and  lungs  in  rapid  alternation  ; 
"  for  a  certain  time  they  inhale  the  air  at  the  surface 
of  the  water,  forming  a  hollow  tube  by  incurving  the 
margin  of  the  mantle,  so  that  the  hollow  surface  is 
enclosed  against  the  water,  and  open  only  at  the  top. 
When  they  have  thus  sucked  in  a  sufficient  quantity 
of  air,  they  reverse  the  margin  of  the  mantle,  opening 
the  tube,  into  which  the  water  streams.  The  changes 
are  tolerably  frequent,  once  or  twice  in  a  few  minutes, 
depending,  probably,  on  the  temperature.  No  phy- 
siological explanation  of  these  rhythmic  alterations 
can,  however,  be  at  present  assigned." 

It  is  not  only  among  the  Mollusca  that  we  have 


Chap,  vi.]  LUNGS  OF  CRUSTACEA.  229 

air-breathing  forms  closely  allied  to  those  that  breathe 
oxygen  dissolved  in  water ;  not  only  are  there  true 
amphipnous  Vertebrates  (see  page  236),  but  there  are 
among  the  Crustacea  some  terrestrial  Isopods,  in 
which  some  of  the  appendages  are  placed  in  a  cavity 
of  the  abdomen,  which  is  partly  closed ;  the  cavity  of 
such  of  these  appendages  as  are  not  rudimentary 
opens  to  the  atmosphere  by  a  longitudinal  slit.  Among 
the  true  crabs  there  are  also  some  forms  that  con- 
stantly live  on  land,  such  as  the  robber  crab  (Birgus 
latro),  the  land  crab  (Gecarcinus),  and  others ;  the 
gills  in  the  branchial  chamber  of  these  Crustacea  are 
always  small,  but  a  quantity  of  air  is  to  be  found  in 
the  chamber ;  in  Birgus  this  chamber  is  divided  into 
a  lower  and  smaller  one,  which  contains  small 
gills,  and  an  upper  larger  one,  which  never  con- 
tains any  water,  but  always  air,  and  which  has 
its  walls  not  only  richly  supplied  with  blood-vessels, 
but  also  produced  into  branched  outgrowths,  or  villi, 
in  which  the  blood-vessels  are  particularly  well  de- 
veloped. 

It  is  not  in  the  lower  Metazoa  alone  that  the 
lining  of  the  alimentary  canal  serves  as  a  means  of 
entrance  for  oxygen ;  even  among  the  Vertebrata, 
where,  in  the  higher  forms,  the  respiratory  organs 
(lungs)  are  really  outgrowths  of  the  enteric  tract,  we 
know  of  a  loach  (Cobitis)  which  swallows  air  bubbles  ; 
among  the  Echinoderms,  the  surface  of  the  mucous 
membrane  of  part  of  the  intestine  is,  in  Stichopus 
variegatus,  so  grooved  as  to  display  a  large  amount  of 
vascular  surface  to  the  action,  of  the  inflowing  water, 
or,  as  in  many  (Holothuria,  Cucumaria),  special 
branched  organs,  which  extend  throughout  the  greater 
part  of  the  length  of  the  body,  are  developed  from  the 
walls  of  the  cloaca ;  in  such  pneumonophorous  holo- 
thurians  water  is  pumped  in  and  out  by  the  muscles 
at  the  hinder  end  of  the  body.  Such  a  form  of  anal 


230  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

respiration  is  not  confined  to  the  Echinodermata,  for 
it  is  very  common  among  the  Arthropoda ;  among  the 
Entomostraca  it  has  been  frequently  observed;  in 
Leptodora,  when  the  intestine  is  free  of  food,  the 
water  appears  to  pass  in  a  continuous  stream  into  the 
more  anterior  parts  of  the  tract,  and,  when  the 
stomach  is  full,  water  is  taken  in  and  expelled  by  the 
mouth ;  anal  respiration  has  been  seen  by  Lereboullet 
in  the  crayfish,  and  the  rhythmical  closing  and 
dilatation  of  the  rectum  may  always  be  seen  in  that 
animal,  after  the  extirpation  of  the  thoracic  ganglia. 

Among  Worms,  enteric  respiration  obtains  in  the 
Rotatoria,  in  some,  if  not  all,  Gephyrea,  and  in  some 
of  the  aquatic  Oligochseta ;  Dentalium  is  the  only 
mollusc  in  which  it  has  been  definitely  observed. 

In  certain  polychsetous  Annelids,  a  somewhat 
complex  arrangement  has  been  detected  by  Eisig, 
which  is  of  especial  interest,  both  morphologically  and 
physiologically,  when  it  is  compared  with  the  function 
and  structure  of  the  respiratory  organs  of  Vertebrates. 
The  observation  that  a  number  of  air  bubbles  constantly 
escape  from  the  mouth  or  anus  of  Hesione  sicula  led 
him  to  detect  the  presence  of  distinct  outgrowths  of 
the  intestine,  which  clearly  serve  as  air  reservoirs,  and 
at  the  same  time  may  be  used  as  floats  or  hydrostatic 
supports  ;  they  are  especially  of  use  when  the  intestine 
is  filled  with  food,  and  structural  evidence  in  favour 
of  their  respiratory  significance  is  afforded  by  their 
rich  supply  of  blood-vessels.  In  connection  with  this 
it  is  very  interesting  to  note  that  a  fish  will  use  up  all 
the  air  in  its  air  bladder  before  it  dies  of  suffocation, 
and  that,  conversely,  in  the  pulmonate  Yertebrata  the 
lungs  have  undoubtedly  the  power  of  acting  as 
hydrostatic  supports  for  the  body,  when  immersed  in 
water. 

The  Cliordata  present  us  with  the  most  interest- 
ing and  instructive  series  of  arrangements,  for  while 


Chap,  vi.j  CHORDATA.  231 

the  lowest  members  of  all  three  divisions  are 
branchiate,  the  higher  Vertebrata  pass  from  an 
amphibious  or  amphipnous  stage  to  one  in  which 
outgrowths  of  the  enteric  tract,  or  lungs,  are  alone 
the  respiratory  organs.  In  the  Ceplialochordata 
and  Urocliordata  respiration  is  always  effected  by 
gills,  and  there  are  some  very  striking  points  of 
agreement  between  the  two  sets  of  forms. 

In  both,  the  gill  slits  are  formed  by  an  ingrowth 
from  without,  and  an  outgrowth  from  within ;  in  both 
it  is  the  anterior  portion  of  the  enteric  tract  which  is 
so  affected,  and  in  both  the  water  of  respiration  enters 
by  the  same  orifice  as  the  food.  In  those  Tunicata 
that  retain  the  cliordate  tail  throughout  life  (Appendi- 
cularia),  the  water  that  passes  in  at  the  mouth  passes 
out  by  a  cylindrical  tube  on  either  side ;  but,  on  the 
other  hand,  water  may  enter  by  these  "  spiracula " 
and  pass  out  by  the  mouth. 

In  the  rest,  as  also  in  Amphioxus,  an  outgrowth 
of  the  body  wall  on  either  side  gives  rise  to  the 
formation  of  a  peribranchial  chamber,  into  which  the 
water  streams  from  the  gills;  the  folds  which  form 
the  walls  of  this  chamber  unite  along  the  greater  part 
of  their  length,  but  leave  an  orifice  (atriopore) 
by  which  the  water  can  escape  to  the  exterior.  This 
atriopore  may  either  open,  as  in  Ascidia,  close  to  the 
incurrent  orifice  or  mouth,  or  it  may  be  at  the  aboral 
end  of  the  body,  as  in  Pyrosoma ;  in  compound 
ascidians  there  is  a  single  common  excurrent  orifice. 
In  some  the  water  is  forcibly  driven  out,  and  then, 
just  as  in  Cephalopoda,  the  excurrent  stream  aids  in 
locomotion.  In  Amphioxus  the  atriopore  is  on  the 
ventral  surface,  and  not  far  in  front  of  the  anus. 

The  ancestors  of  the  present  race  of  Vertebrata 
were  aquatic  forms  that  breathed  the  oxygen  which 
was  dissolved  in  the  water  in  which  they  dwelt  ; 
or  in  other  words,  they  had  gills.  This  mode  of 


232  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

respiration,  the  branchial,  is  retained  to-day  by  the 
lowest  of  the  Vertebrata,  and  gills  are  to  be  found  in 
all  Fishes,  in  all  Amphibia  at  some  period  of  their  lives, 
and  in  some  Amphibia  throughout  the  whole  course  of 
their  existence.  None  of  the  Sauropsida  or  of  the 
Mammalia  ever  breathe  oxygen  dissolved  in  water, 
but'  are  air-breathing  forms  with  lungs ;  though  the 
change  of  function  has  been  completed,  the  remnants 
of  gill  clefts  are  observable  in  the  earlier  stages  of 
development 

A  most  instructive  series  of  gradations  is  to  be 
detected  in  Fishes;  all  adult 'forms  have  the  gills  in 
pouches  or  recesses,  but  the  young  of  some  (Elasmo- 
branchs,  some  Ganoids),  like  the  tadpole  at  an  early 
stage,  have  protruding  filaments  or  external  gills ; 
the  lampreys  have  seven  pairs  of  gill-clefts,  as  has  the 
shark  Heptanchus ;  Hexanchus,  and  most  examples  of 
Myxine,  have  six  ;  most  Elasmobranchs,  five  pairs  ; 
Chimsera  has  the  first  and  fifth  gill  incomplete  ;  most 
Teleosteans  have  four  pairs  of  gills,  but  some  have  the 
fourth  incomplete ;  the  angler  (Lophius)  has  three 
pairs  of  gills,  its  ally  Malthe  has  the  third  incomplete, 
while  in  Amphipnous  all  the  three  pairs  of  gills  are 
more  or  less  rudimentary. 

Where  gill  respiration  ceases  to  be  effective,  the 
blind  outgrowth  from  the  anterior  portion  of  the 
intestine  (the  air  bladder)  may  take  on  respiratory 
functions ;  Amia  has  a  single  sac  lying  on  the  dorsal 
surface  of  the  intestine  ;  in  Lepidosteus  (the  gar-pike), 
the  sac  is  divided  internally,  though  it  is  single 
externally ;  in  Ceratodus  the  opening  into  the  single 
sac  lies  to  the  left  of  the  ventral  surface  of  the 
intestine,  while  in  Polypterus  the  sac  is  double,  and 
opens  on  the  middle  line  of  the  ventral  surface.  As 
the  air  bladder  becomes  better  developed,  it  becomes 
better  supplied  with  blood.  (See  page  202.) 

In  the  cyclostomatous  Bdellostoma  the  ducts  from 


Chap.  VI.] 


GILLS  OF  FISHES. 


233 


the  gill  sacs  open  separately   to  the  exterior,  but  in 

the  hag  the  ducts  unite  and  open  by  a  common  orifice 

on  either  side.     A  similar  modification  obtains  in  the 

gnathostomatous    fishes,    for   in    the 

shark -like  elasmobranchs  (sharks  and 

rays)  each  gill  cleft  is  open  to  the 

exterior,  while  in  Chimsera  and  all 

other  Fishes  the   clefts  are  covered 

over  either  by  a   fold  of   the   skin 

merely,  or  by  bony  pieces  (opercu- 

liini),  so  that  there  is  on  either  side 

but  a  single  opening  to  the  exterior. 

The  gills,  when  complete,  consist 
of  two  folds  of  mucous  membrane, 
which  are  ordinarily  triangular  in 
shape  (Fig.  98),  are  supported  by  a 
branchial  arch  (b),  and  by  cartila- 
ginous rods,  have  a  blood-vessel 
passing  into  them  and  bringing  blood 
from  the  heart,  which  breaks  up 
into  capillaries ;  these  capillaries 
unite  into  another  vessel  which  car- 
ries the  oxygenated  blood  back  to 
the  body.  A  gill  is  said  to  be 
incomplete  when  one  of  the  two 
folds  is  alone  developed. 

IntheCyclostomata  and  the  shark  • 
like  Elasmobranchs,  the  gills  take  the 
form  of  pouches,  and  the  lamellae 
form  the  transverse  folds  on  the 
walls  of  the  pouch ;  the  septa  which 
project  from  the  branchial  arches  are 
as  long  as  the  gills,  and  each  gill 
chamber  is,  therefore,  in  a  shark,  completely  separated 
from  the  one  in  front  or  behind  it ;  as  we  pass  through 
Chimaera  and  the  Ganoids  to  the  Teleostei,  we  find 
that  these  septa  become  more  and  more  reduced,  so  that 


the  form  of  the 
gills  and  the  ar- 
rangement of  the 
blood-vessels. 

a,  Branchial  artery ; 
6,  branchial  arch 
(seen  in  cross  sec- 
tion); c,  branches  of 
the  branchial  vein 
v  ;  d,  branches  of 
branchial  artery. 


234  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  gill  is  attached  at  its  base  only  (Fig.  98),  and  the 
gill  lamellae  are  free.  With  the  disappearance  of  the 
septa  we  have,  of  course,  the  loss  of  the  separate  gill 
slits,  and  the  whole  of  the  gills  of  one  side  come  to 
lie  in  a  common  chamber,  which  is  covered  over  by 
the  operculum,  and  has  only  one  opening  to  the 
exterior. 

The  water  which  brings  the  necessary  oxygen  to 
the  gills  enters  by  the  mouth ;  as  the  mouth  opens 

the  operculum 
rises,  and  the 
gills  separate 
from  one  an- 
other, but  the 
membrane  which 
fringes  the  oper- 
culum acts  as  a 
valve  to  prevent 
the  entrance  of 
water  through 
the  opercular 
cleft  (Bert). 
When  the  mouth 
closes,  and  the 
pharynx  con- 
tracts, the  water  is  forced  through  the  pharyngeal 
clefts  into  the  gill  clefts  owing  to  the  presence  of  a 
valvular  arrangement  which  shuts  off  the  passage  into 
the  mouth. 

In  the  already  mentioned  Amphipnous  and  in 
Saccobranchus,  the  true  gills  are  rudimentary,  and  a 
sac  with  contractile  walls  is  developed,  which  takes 
in  water  and  expels  it  at  intervals ;  the  walls  of  these 
sacs  are  richly  supplied  with  blood-vessels,  which  are 
arranged  as  in  a  gill ;  that  is  to  say,  the  blood  that 
passes  from  them  goes  direct  to  the  aorta ;  in  the 
climbing  perch  (Anabas),  the  internal  surface  of  the 


Fig.  99. — Suprabrancbial  organ  of  Andbas 

scandens. 
a,  Supraforanchial  organ. 


ehap.  vi.]  AIR  SACS  OF  FISHES.  235 

accessory  sac  is  produced  into  a  number  of  convoluted 
folds  (Fig.  99),  which  retain  their  moisture,  and  are 
able  to  take  up  oxygen  direct  from  the  air,  during  the 
comparatively  long  periods  that  the  fish  lives  out  of 
the  water.  Its  ally  Ophiocephalus,  Cobitis,  and 
various  fishes  with  spongy  air-bladders,  such  as  Sudis 
and  Erythrinus,  swallow  air  directly ;  so  that  it  is  not 
among  the  Dipnoi  only  that  oxygen  dissolved  in 
nitrogen  (atmospheric  air),  is  used  for  the  necessary 
oxydation  of  the  tissues  of  fishes. 

It  is,  however,  in  certain  Ganoids  and  in  the 
Dipnoi  that  we  get  the  most  certain  proofs  of  aerial 
respiration ;  Lepidosteus  has  been  observed  to  pro- 
trude its  head  from  the  water,  to  emit  a  bubble  of 
air,  and  to  make  a  swallowing  movement,  and  a 
similar  phenomenon  has  been  seen  in  Araia  (B.  G. 
Wilder) ;  the  noise  made  by  Ceratodus  is  explained  as 
being  due  to  the  swallowing  of  air,  and  the  streams  of 
Australia  in  which  it  lives  are  known  to  become 
liquid  mud  in  the  dry  seasons  of  the  year.  Protopterus 
has  been  brought  from  West  Africa  to  this  country 
embedded  in  the  mud  balls  in  which  it  lives  during 
the  droughts,  and  has  been  revived  by  being  placed  in 
warm  water. 

Fishes  differ  considerably  in  the  extent  to  which 
they  are  able  to  live  on  land ;  thus,  an  eel  will  live 
much  longer  than  a  gudgeon  when  taken  out  of  the 
water.  The  careful  experiments  of  Bert  show  that 
this  difference  is  due  not  to  a  difference  in  gill  arrange- 
ment, but  to  a  difference  in  the  demand  made  by  the 
tissues  of  the  body  for  their  supply  of  oxygen.  Here, 
again,  we  have  an  example  of  the  danger  of  arguing 
from  anatomical  peculiarities  where  our  hypotheses 
are  not  controlled  by  experiments.  In  any  discussion 
of  respiratory  phenomena  in  animals  it  is  neces- 
sary to  bear  in  mind  the  fact  that  all  living 
tissues  are  capable  of  absorbing  oxygen,  and  that 


236  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  tissues  of  different  animals  differ  in  the  amount 
they  require. 

We  may  be  especially  convinced  of  the  truth  of  this 
dictum  of  Bert's  by  the  study  of  the  respiratory  arrange- 
ments of  the  Amphibia.  If  an  adult  frog  is  placed  in  a 
dry  atmosphere  it  speedily  dies ;  in  other  words,  respira- 
tion in  a  frog  is  cutaneous  as  well  as  pulmonary,  and  a 
frog  may  be  deprived  of  its  lungs  and  continue  to  live  ; 
so,  again,  the  axolotl  may  have  both  gills  and  lungs 
removed  and  yet  continue  to  live.  But  if  these 
experiments  are  made  in  summer  death  soon  super- 
venes ;  in  other  words,  the  skin  becomes  more  dry 
owing  to  the  larger  amount  of  moisture  which  can  be 
taken  up  by  an  equal  volume  of  warmer  air,  and  is 
unable  to  take  up  enough  oxygen  to  suffice  for  the 
needs  of  the  organism. 

In  all  Amphibia  the  gills  are  at  first  external,  or 
project,  under  various  forms,  from  the  sides  of  the 
body  ;  there  are  ordinarily  three  pairs  present,  which 
are  placed  one  above  the  other ;  among  the  Urodela, 
Menobranchus  (Fig.  100)  and  Proteus  appear  to 
retain  the  gills  throughout  life;  in  Menopoma  and 
Amphiuma  the  gills  disappear,  but  one  or  two  gill 
clefts  persist ;  in  the  rest  of  the  Urodela  the  gills  dis- 
appear completely.  In  the  Anura  the  external  are 
soon  replaced  by  internal  gills,  which,  on  the  cessation 
of  the  tadpole  stage,  disappear,  and  the  clefts,  which 
had  been  covered  by  an  opercular  membrane,  close  up 
entirely.  The  bell-shaped  gills  of  Notodelphys  lead  to 
the  branchial  vesicles  which  have  been  found  in  the 
Cacciliac  (Peters). 

In  addition  to  or  in  place  of  the  gills,  all  Amphibia 
have  a  paired  outgrowth  from  the  oesophagus,  which 
lies  on  the  ventral  surface,  and  is  provided  with 
special  blood-vessels  coming  directly  from  and  return- 
ing at  once  to  the  heart. 

These  outgrowths  are  known  as  the  lungs,  and 


Chap.  VI.] 


ME  NO  BRA  NCH  US. 


237 


238  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

they  seem  to  be  the  direct  descendants  of  the  swim- 
bladder  of  fishes,  which  we  have  already  traced  from 
its  position  as  a  dorsal  sac  in  Amia  and  Lepidosteus 
to  a  completely  divided  ventral  sac  in  Protopterus. 


PI) 


PD 


PD 


Fig.  101. — Diagrams  to  illustrate  the  Development  of  tbe  Lungs. 

PD,  Primitive  intestine;  ss',  lung  sacs;  t,  trachea;  b,  bronchus  (.in  A,  B,  and 
c);  L0  (in  D),  primitive ;  i,g  (in  a),  secondary  pulmonary  vesicles.  (After 
Wiedersheim.) 

Comparable  with  these  changes  in  the  coarser  details 
of  its  anatomy  are  the  modifications  suffered  by  its 
internal  surface,  which  becomes  more  and  "more  spongy 
and  broken  up  into  internal  spaces ;  and  the  changes 
which  bring  its  blood-vessels  into  direct  relation  with 
the  heart.  (See  page  203.) 

A  similar  set  of  changes  affects  the  lungs,  either 
as  we  trace  them  through  the  ascending  scale  of  the 


chap.  vi. j          LUNGS  OF  VERTEBRATES.  239 

pentadactyle  Vertebrata,  or  through  the  developmental 
stages  of  a  given  individual.  The  earliest  rudiment  of 
the  lung  is  a  single  outgrowth  (Fig.  101 ;  A),  which  soon 
divides  at  its  blind  end  (B),  while  the  unpaired 
portion  remains  to  form  the  tube  (trachea)  by 
which  the  two  sacs  communicate  with  the  oesophagus; 
each  swelling  gives  rise  to  primary  (D),  and  these 
to  secondary  (E)  vesicles. 

This  series  of  changes  ceases  at  various  points  in 
various  forms,  so  that  the  lungs  are  smooth  within  in 
Menobranchus,  provided  with  a  few  simple  ridges  in 
Siren,  and  with  secondary  as  well  as  primary  ridges  in 
Amphiuma.  The  internal  network  in  which  the 
blood-vessels  course  is  still  more  elaborately  developed 
in  the  frog,  but  the  lungs  are,  when  looked  at  from 
without,  apparently  nothing  more  than  simple  sacs. 

The  same  is  true  of  the  lower  Kept  ilia;  but  there 
is  this  important  advance,  that  the  bronchus,  or  tube 
which  brings  the  air  into  the  lungs,  does  not,  as  in  the 
frog,  cease  at  the  opening  into  the  lung,  but  is  con- 
tinued into  it,  and  gives  off  branches  within  it ;  in 
some  chamseleons  narrow  blind  outgrowths  proceed 
from  the  hinder  end  of  the  lung,  and  in  Chelonians 
and  Crocodiles  the  common  lung-chamber  opens  into  a 
number  of  pouch-like  sacs.  The  lungs  of  the  former, 
like  those  of  birds,  are  firmly  attached  on  either  side 
of  the  vertebral  column,  and  the  dorsal  surface  is 
marked  by  grooves  which  correspond  in  position  to 
those  of  the  superjacent  ribs. 

The  lungs  of  Birds,  in  addition  to  their  greater 
internal  complexity,  are  more  particularly  remarkable 
for  being  continued  into  a  number  of  air  sacs, 
whence  prolongations  are  given  off  in  the  form  of  air 
tubes  and  passages,  which  extend  through  all  the 
organs,  including  even  the  bones,  of  the  body.  These 
air  sacs  play  a  very  important  part  in  the  economy  of 
the  bird,  for  they  not  only  diminish  its  specific  gravity, 


240  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

but  also  warm  the  air.  It  has  been  calculated  by- 
Bert  that,  in  a  bird  weighing  1,600  grammes,  and 
having  a  volume  equal  to  1,230  cubic  centimetres,  or, 

in  other  words,  a  specific  gravity  of  1  *3  (—  — ),  200 

\1230 ' 

cubic  centimetres  of  air  can  be  introduced  ;  as  these 
200  cubic  centimetres  weigh  -22  of  a  gramme,  it  is  clear 
that  the  specific  gravity  of  the  animal  will  be  reduced 

-  10  /1600  +  0-22         1600-22V,, 
10  1-1S  (l230  +  200  OT  1«T>* 

must  often  take  into  their  lungs  air  at  a  very  low 
temperature,  but  with  this  cold  air  there  is  com- 
mingled that  which  returns  to  the  lungs  from  the 
warm  viscera,  and  by  this  means  the  temperature  of 
the  respired  air  is  raised ;  yet  again,  such  cold  air,  or, 
still  more,  the  air  of  a  desert,  is  often  of  great  dryness, 
while  that  which  returns  from  the  air  sacs  has  been 
moistened  by  the  walls  of  these  outgrowths. 

The  maximum  of  complexity  is  attained  by  the 
lungs  of  the  Mammalia,  which,  occupying  a  com- 
paratively smaller  space  in  the  body,  have  nevertheless 
a  much  larger  area  of  respiratory  surface  ;  externally 
the  lungs  are  frequently  subdivided  into  two  or  more 
lobes.  It  has  been  calculated  by  Aeby  that  the 
human  lung  contains  from  three  to  four  millions  of 
pulmonary  vesicles,  and  that  in  man  the  respiratory 
mucous  membrane  has,  in  a  period  of  repose,  a 
superficial  area  of  79 '28  square  metres,  which 
can  be  extended  to  more  than  half  as  much  again, 
or  129 '8 4  square  metres;  the  extent  of  respiratory 
surface  in  the  female  is  rather  less  than  that  of 
the  male. 

The  air  is  brought  into  the  lungs  from  the  nasal 
passages  by  the  trachea,  and  that  tube,  as  we  know, 
divides  into  two  bronchi,  which,  in  the  Amniota, 

*  I  have  corrected  what  appears  to  be  an  error  in  Bert's  calcu- 
lation. 


chip,  vi.]  BRONCHI  ;  TRACHEA.  241 

extend  into  the  cavity  of  the  lung.  The  bronchi  are 
short  indeed  in  lizards  and  snakes,  but  in  crocodiles 
and  chelonians  they  extend  for  some  considerable  dis- 
tance, and  retain  the  cartilaginous  rings  by  means  of 
which  the  tube  is  kept  open  ;  these  tubes  give  off 
smaller  lateral  tubes,  and  so  give  rise  to  the  so-called 
bronchial  tree,  some  of  the  branches  of  which  lie 
above  and  some  below  the  pulmonary  artery  (p  ;  Fig. 
102  A)  ;  these  may  be  conveniently  distinguished  as  the 
eparterial  and  hyparterial  bronchi. 

While  this  bronchial  tree  is  comparatively  simple 
in  Reptiles,  it  becomes  much  more  complicated  in  Birds, 
where  both  eparterial  and  hyparterial  systems  are 
well  developed  and  give  off  lateral  branches,  some  of 
which  extend  to  the  end  of  the  lung. 

The  difference  between  the  bronchial  tree  of  a  Bird 
and  a  Mammal  does  not  lie  in,  as  is  ordinarily  said, 
the  dichotomous  mode  of  division  of  the  bronchial 
tubes  of  the  latter,  which  never  does  obtain  (Aeby), 
but  in  the  great  reduction  in  the  eparterial  bronchial 
system  (Fig.  102  B),  of  which  the  right  and  left  halves 
are  but  rarely  both  present ;  as  a  rule,  the  right  is 
lost,  while  in  Hystrix  both  right  and  left  eparterial 
bronchi  disappear. 

The  trachea  varies  greatly  in  the  extent  and 
characters  of  its  development ;  short  in  Amphibians,  it 
is  of  a  considerable  length  in  Reptiles,  but  in  them  the 
cartilaginous  rings  are  incomplete  ;  in  Mammals  the 
rings  are  also  always  incomplete,  but  in  Birds  the  sepa- 
rate rings  are  not  only  complete,  but  tend  to  undergo 
calcification,  and  in  some  cases,  as  in  the  Dinornis, 
even  ossification.  The  trachea  is  of  great  length  in 
birds,  and  while  this  may  be  often  seen  to  be  of 
significance  as  an  aid  to  the  vocal  organ  (see  page  391), 
it  has  clearly  the  not  unimportant  function  of  forming 
a  long  tube  in  which  the  air  is  slightly  warmed  before 
it  enters  the  lungs.  In  birds  the  lower,  and  in  other 
Q-16 


242    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Vertebrates  the  upper,  part  of  the  trachea  may  be  con- 
verted into  a  vocal  organ. 

In  the  skulls  of  certain  Vertebrates,  such  as  the 
crocodile  and  the  whale,  certain  modifications  of  the 

bones  of  the  palate 
bring  about  an  elon- 
gation of  the  nasal 
passages  and  an  ap- 
proximation of 
posterior  nares 
the  opening  of 
v  /jr  \m  trachea  (see 


the 

to 

the 

page 


344)  ;  by 
these  means 
water  is  pre- 
vented, at 
least  in  part, 


lungs.  other  adaptive 
modifications  to  the 
same  purpose  may  be 
conveniently  consi- 
dered here. 

In  the  Whales  the 
glottis,  or  opening 
into  the  trachea,  is 
produced  into  a  fun- 
nel -  like  projection, 
which  extends  into 
the  soft  palate,  and  is 

embraced  by  its  sides.  By  this  means  the  trachea  is 
brought  into  direct  connection  with  the  nasal  passages, 
the  air  does  not  enter  at  all  into  the  cavity  of  the 
mouth,  and  the  water  flows  on  either  side  into  the 
gullet.  A  similar  disposition  of  the  glottis  obtains 
in  the  young  of  the  Marsupials,  which,  born  at  an 


B 


Fig.  102  A.— Bronchial  Tree  of  a  Bird. 

p,  Pulmonary  artery  ;  A,  eparterial ;  B,  hyp? 
term    bronchial  systems  ;  v,  ventral  ; 
dorsal  branches. 


Chap.  VI.] 


CETACEA  ;  SIREWA. 


243 


Here,    then,    milk 


B 


age  too  early  to  allow  them  to  actively  suck  the 
mother,  hang  on  to  a  long  nipple,  and  have  the 
milk  injected  by  the  mother  (by  the  contraction  of 
the  cremaster  muscle,  and  the  consequent  compres- 
sion of  the  mammary  glands), 
flows  on  either  side  of  the 
air  tube,  and  the  latter  is, 
as  in  the  whale,  a  direct 
continuation  of  the  air 
passages  in  the  head.  It 
is  important  to  observe  that 
•there  is  no  prolongation  of 
the  air  tube  in  the  Sirenia, 
but  that  their  epiglottis  is 
large,  and  capable  of  com- 
pletely closing  the  entrance 
into  the  trachea;  at  the 
same  time  it  will  be  remem- 
bered that  the  dugong  and 
manatee  are  herbivorous. 

So,  again,  the  Sirenia 
differ  from  the  Cetacea  in 
the  manner  in  which  they 
obtain  a  large  supply  of  air. 
In  the  former  the  dia- 
phragm, in  place  of  forming 
a  more  or  less  vertical  par- 
tition between  the  thoracic 
and  abdominal  cavities,  slopes  backwards  and  upwards, 
so  as  to  largely  increase  the  area  of  the  thoracic  cavity, 
the  extension  of  which  is  occupied  by  the  large  lungs. 
In  the  dolphins  and  porpoises  the  nasal  passages  open 
into  lateral  sacs  with  elastic  walls  ;  the  possession  of.' 
these  sacs  must,  in  addition  to  their  air-containing 
function,  diminish  to  a  certain  extent  the  specific 
gravity  of  the  skull.  The  commonly  received  story 
that  a  whale  "  blows  "  water  is  due  to  the  fact  that  a 


Fig.  102  B.— Bronchial  Tree  of  a 
Mammal  (Horse). 

A,  Eparterial :  B,  hyj>arterial  ventral 
(»1; (<f)  hyparterial  dorsal  bronchi; 
pa,  pv,  pulmonary  artery  and 
vein.  (After  Aeby.) 


244    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

large  quantity  of  warm  air  is  rapidly  expelled  through 
the  single  spiracle  (the  homologue  of  the  two  external 
nares  of  other  animals),  or  so-called  "blow-hole,"  and 
that  the  moisture  in  this  air  condenses  into  water 
as  it  suddenly  comes  into  a  colder  medium.  A  whale 
no  more  breathes  water  than  does  a  man  on  a  frosty 
day. 

Pulmonary  respiration,  or  the  taking-in  and  the 
expulsion  of  air  from  the  lungs,  is  effected  in  very 
various  ways  in  different  Vertebrates ;  the  air  tube 
being,  as  we  know,  ordinarily  kept  open  by  the  car- 
tilaginous or  bony  rings  or  supports  which  are  found 
in  its  walls,  it  is  clear  that  air  may  either  be  driven 
out  or  sucked  in. 

In  the  Dipnoi  there  is  no  cartilaginous  trachea, 
and  the  air  enters  in  and  passes  out  by  a  longitudinal 
slit,  the  sides  of  which  are  separated  from  one  another 
by  the  contraction  of  the  muscle  that  surrounds  it. 
In  the  Perennibranchiata  there  is  no  true  trachea,  but 
on  either  side  of  the  slit  there  is  a  small  cartilage 
with  which  a  constrictor  or  a  dilatator  muscle  is 
connected  (Wiedersheim).  In  the  rest  of  the  Am- 
phibia there  is  a  true  trachea  of  no  great  length,  the 
opening  into  which  is  sometimes  provided  with  muscles, 
by  means  of  which  it  can  be  enlarged  or  diminished 
in  size.  In  the  frog,  whose  physiology  has  been  more 
fully  studied,  air  is  known  to  be  forced  into  the  lungs 
by  the  action  of  the  muscles  of  the  floor  of  the  mouth  ; 
this  apparatus,  appropriately  known  as  the  buccal 
pump,  acts  in  the  following  manner  :  the  mouth  is 
shut,  and  the  floor  of  the  mouth  depressed  by  the 
contraction  of  the  muscles  connected  therewith ;  the 
vacuum  so  formed  is  filled  by  the  entrance  of  air 
through  the  nostrils  and  nasal  passages ;  the  nostrils 
are  then  closed,  and  the  entrance  to  the  gullet  barred, 
while  the  floor  of  the  mouth  rises  on  the  contraction  of 
the  muscles  connected  with  the  hyoid ;  the  entrance 


Chap.  VI.]  Rp.PTILES ;   BlRDS.  245 

to  the  air  tube  is  widened  by  the  contraction  of  the 
dilatator  muscles,  and  the  compressed  air,  finding 
there  its  only  means  of  exit,  enters  the  passage  to  the 
lungs.  By  the  elasticity  of  the  walls  of  the  lungs 
themselves,  and  by  the  contraction  of  their  muscles 
and  those  of  the  body  wall,  the  air  that  has  thus 
entered  is  soon  afterwards  driven  out. 

In  the  Chelonia,  which,  in  accordance  with  their 
sluggish  habits,  execute  respiratory  movements  only 
three  times  a  minute  (Bert),  the  thorax  is  dilated  by 
a  special  inspiratory  muscle,  and  the  limbs  only  take 
part  in  the  action  when  inspiration  and  expiration 
succeed  one  another  with  more  than  an  ordinary 
rapidity.  In  the  Ophidia  the  cavity  of  the  thorax  is 
increased  by  the  movements  of  the  ribs,  and  as  these 
are  also  the  locomotor  organs  of  snakes,  we  have  here 
again  an  example  of  the  relation  between  respiratory 
and  locomotor  activity.  In  Lizards  and  Crocodiles, 
where  the  belly  ordinarily  touches  the  ground,  the 
thorax  is  extended  transversely  much  more  than  from 
above  downwards,  for  in  all  Amniota  the  enlargement 
of  the  cavity  is  effected  by  the  movements  of  the  ribs. 
The  expulsion  of  the  air  is  brought  about  by  the  con- 
tractility of  the  walls  of  the  lungs. 

In  Birds  the  lungs  are  fixed  to  the  back  and  sides 
of  the  thorax,  the  extension  of  which,  in  the  movements 
of  expiration,  is  much  greater  in  the  vertical  than  in 
the  transverse  direction.  An  inspection  of  the  skele- 
ton of  a  bird  (Fig.  135)  will  show  that  the  ribs  con- 
nected with  the  spinal  column  are  set  at  an  angle  to 
those  which  are  connected  with  the  sternum ;  on  the 
contraction  of  the  inspiratory  muscles  this  angle 
becomes  more  open,  the  sternum  is  more  widely 
separated  from  the  back,  and  the  thoracic  region  is 
increased  in  extent ;  there  is,  at  the  same  time,  a 
certain  amount  of  transverse  extension.  When  the 
thorax  enlarges  air  is  drawn  in  from  the  air  sacs  as 


246    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

well  as  from  the  outer  world  ;  and  when  the  thorax 
contracts,  in  the  act  of  expiration,  air  is  driven  into 
the  air  sacs  as  well  as  through  the  trachea  outwards. 

In  Mammals  the  movements  of  the  ribs  are 
greatly  aided  by  the  flattening  out,  or  curving  up- 
wards, of  the  diaphragm,  or  muscular  partition 
which  separates  the  thoracic  from  the  abdominal 
portion  of  the  body  cavity.  The  respiratory  move- 
ments of  mammals  have  been  fully  studied  in  Man. 
(See  "  Human  Physiology,"  chap,  v.) 

Bert  has  collected  a  large  number  of  statistics 
with  regard  to  the  number  of  respiratory  move- 
ments executed  per  minute  by  various  animals. 
From  this  we  learn  that,  on  the  whole,  they  are 
more  numerous  in  Mammals  than  in  Birds.  A  rat, 
for  example,  has  been  seen  to  make  320  movements 
a  minute,  while  the  canary  gives  the  highest  (100) 
number  for  birds.  Rodents  generally  respire  fre- 
quently ;  the  dog  and  ox  15  times,  the  lion  and  horse 
10,  and  a  hippopotamus  was  on  one  occasion  observed 
to  breathe  only  once  in  a  minute ;  some  large  birds, 
such  as '  the  marabou,  pelican,  or  condor,  only  4  to  6 
times  a  minute;  a  Crotalus  5  times,  a  lizard  12.  An 
active  sea- lamprey  gave  a  number  of  120  ;  rays  and 
dog-fishes  from  40  to  50,  Limulus  12,  while  Cepha- 
lopods  varied  between  14  and  65  times  a  minute.  On 
the  whole,  carnivorous  breathe  less  frequently  than 
herbivorous  forms,  and  both  than  rodents ;  smaller 
forms  more  frequently  than  larger  members  of  the 
same  group,  and  active  more  often  than  sluggish 
species.  It  is,  however,  to  be  carefully  observed 
that  these  numbers  give  us  no  information  as  to  the 
quantity  of  air  taken  in,  nor  as  to  the  number  of 
times  in  which  the  heart  was  beating  per  minute; 


247 


CHAPTER  VII. 

ORGANS  OF  NITROGENOUS  EXCRETION. 

VERY  much  doubt  hangs  over  the  function  of  the 
organs  which  are  said  to  have  a  renal  function  in  the 
lower  animals,  owing  to  the  great  discrepancies  be- 
tween the  results  attained  to  by  those  who  have  inves- 
tigated the  excreta  of,  or  concretions  in,  these  organs, 
and  the  very  great  difficulties  which  lie  in  the  way  of 
such  chemical  inquiries. 

Nothing  can  be  certainly  said  as  to  the  renal 
organs  of  the  Protozoa,  if  we  may  use  the  term 
renal  organs  in  a  general  way,  as  applying  to  such 
parts  of  the  organism  as  purify  the  body  of  its  nitro- 
genous waste  ;  this,  however,  is  certain,  that  in  the 
course  of  the  molecular  activity  of  a  mass  of  proto- 
plasm, nitrogenous  products  are  formed  which  are  in 
the  nature  of  waste  products,  and  which  are  injurious 
to  the  organism  if  not  speedily  removed  from  it.  We 
know  that  in  most  Protozoa  there  are  one  or  more 
spaces  which,  expanding,  take  up,  and,  contracting, 
drive  out,  water.  Knowing,  as  wre  do,  from  our  own 
experience,  the  value  of  water  as  a  diuretic  agent,  it 
seems  almost  justifiable  to  suppose  that,  while  this 
water  has  no  doubt  a  respiratory  function  in  the 
Protozoa,  it  acts  also  as  an  agent  for  removing  waste. 
The  supposition  that  the  office  of  the  contractile  vesicle 
is  to  drive  fluid  out  of  the  body  is  supported  by  the 
discovery  of  Vorticellids,  in  which  the  contractile 
vesicle  is  connected  by  a  canal  with  the  "  vestibule  " 
which  lies  beneath  the  mouth  opening ;  on  the  con- 
traction of  the  vesicle  the  contained  water  passes  into 
the  mouth-opening,  and  so,  of  course,  makes  its  way 


248    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

to  the  exterior.  In  the  same  manner  we  may  sup- 
pose that  in  the  Sponges,  which  certainly  do  get  rid 
of  nitrogenous  waste,  the  currents  of  water  that  pass 
through  the  canals  of  the  body  wall  carry  away  with 
them  waste  nitrogenous  formations ;  but  here,  as  in 
the  Protozoa,  experimental  evidence  is  still  wanting. 
We  are  hardly  better  off  for  information  as  concerns 
the  Ccelenterata  or  the  Echiiiodermata.  Of  the 
former  class,  indeed,  the  mesenterial  filaments  of 
Actiniae,  and  a  whitish  layer  on  the  lower  side  of  the 
umbrella  of  Porpita,  have  been  stated  to  contain 
guanin,  which  is  a  waste  nitrogenous  product ;  and 
the  same  compound  has  been  said  to  be  found  in  the 
rectal  caeca  of  the  starfish,  and  in  the  Cuvierian 
organs  of  certain  holothurians ;  as  to  the  last,  how- 
ever, it  is  doubtful  whether  the  true  Cuvierian  organs 
were  really  examined,  and,  as  to  their  function,  the 
great  balance  of  evidence  is  in  favour  of  their  being 
rather  offensive  than  excretory  organs. 

In  the  Verifies  we  have  the  advantage  of  being 
able  to  detect  organs  which,  by  their  position,  rela- 
tions, and  homologies,  afford  considerable  support  to 
the  view  that  they  have  a  renal  function.  It  will  be 
most  convenient  to  first  examine  the  so-called  seg- 
mental  or  kidney -like  organs  (iiephridia)  of  so  well 
developed  a  form  as  the  earthworm. 

In  all  but  the  first  segment  of  the  body  we  find 
on  the  ventral  surface  and  on  either  side  of  the  middle 
line,  a  convoluted  tube,  which  opens  by  a  funnel- 
shaped  orifice  into  the  body  cavity,  penetrates 
the  membranous  wall  which  separates  one  segment 
from  the  next  succeeding,  and  in  the  latter  opens  to 
the  exterior  by  a  small  pore.  The  ciliated  funnel- 
shaped  opening,  and  the  thinner-walled  portion  of 
the  coiled  duct,  may  be  looked  upon  as  the  receiving 
portion  of  the  organ  ;  the  true  excretory  activity  is, 
no  doubt,  limited  to  the  part  where  the  walls  are 


Chap.   VI T.] 


PL  A  TYHELMINTHES. 


249 


glandular,  and  these  glands,  we  may  suppose,  act  on 

the  contents  of   the   blood-vessels  which    are   richly 

distributed   to   the   nephridium.      The  terminal  and 

wider    portion,    the    walls    of 

which    are    muscular,    may  be 

looked  upon  as  analogous  to  a 

ureter. 

Bearing  in  mind  that  we 
have  in  the  earthworm  to  do 
with  a  form  in  which  meta- 
meric  segmentation  is  most 
markedly  expressed,  and  that 
this  metamerism  has  clearly 
affected  the  nephridia,  we  are 
prepared  to  find  a  very  much 
simpler  condition  of  things 
among  the  Platyhelminthes, 
and,  at  the  same  time,  to  find  an 
arrangement  which  is  more  dif- 
fused. InMonoccelis  (Fig.  104), 
for  example,  there  is  a  plexus  of 
fine  canals,  which  communicate, 
on  the  one  side,  with  large 
principal  canals,  of  which  there 
are  two  pairs,  one  external 
and  one  internal,  and  on  the 
other  with  funnel-shaped  pro-  Fig.^ios.— A  single  *:« 
cesses,  the  entrance  to  which  is 
guarded  by  a  long  cilium  ;  the  "' 
principal  canals  are  connected 
with  one  another  by  anastomo- 
sing branches.  In  the  Den- 
drocoela,  as  represented  by 
Polyccelis,  the  fine  canals  appear  to  be  absent. 

If  we  take  the  liver  fluke  as  a  type  of  the 
Trematoda,  we  again  find  that  the  system  of 
excretory  vessels  is  diffused  throughout  the  whole 


dium  of  Anachceto- 


gSTes 


K 


250    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

body ;  the  finest  ducts  are  distributed  through  all 
parts  of  the  organism,  and  they  pass  into  collecting 
vessels,  which,  by  the  formation  of  anastomoses,  give 
rise  to  a  most  complicated  plexus ;  from  these  arise 


Fig.  104. — Excretory  System  of  Monoccelis  fusca,  showing  the  numerous 
Infuudibula,  and  the  brauchiug  Tubes.     (After  Fraipont.) 

the  efferent  ducts,  which  gradually  unite  into  collect- 
ing vessels;  these,  again,  form  a  plexus,  and  from 
these  there  again  arise  vessels  which  pass  into  a 
median  longitudinal  trunk,  which  opens  at  the  hinder 
end  of  the  body  by  an  excretory  pore.  There  are 
no  valves  or  muscular  walls  by  means  of  which  the 
products  are  aided  on  their  way  to  the  outer  world. 
The  contents  of  the  vessels  are  stated  to  be  colourless, 
and  to  contain  a  number  of  small  particles  of  high 
refractive  power ;  Lieberkiihn  says  that  he  has  been 
able  to  detect  the  presence  of  guanin. 

Among  the  Cestoda  we  find  that,  while  the 
young  of  some  forms  have  a  complicated  system  of 
fine  canals,  the  ordinary  arrangement  is  that  of  two 


Chap,  vii.]  CESTODA  ;  ROTATORIA.  251 

longitudinal  vessels  which  extend  through  all  the 
joints,  which  may  or  may  not  branch  and  form  a 
plexus  in  the  head,  and  which  open  to  the  exterior  by 
a  single  excretory  pore,  which  is  placed  in  the  terminal 
joint ;  sometimes  the  tubes  open  in  the  several  joints 
by  secondary  foramina  (Fraipont),  and  in  such  cases 
the  terminal  pore  and  vesicle  become  more  or  less 
atrophied.  These  secondary  orifices  in  a  tapeworm 
are  not  to  be  compared  with  the  openings  of  the 
"  segmental  organs  "  in  the  earthworm.  The  calcareous 
concretions  which  are  so  frequently  observed  in  tape- 
worms have  not  as  yet  been  certainly  shown  to  have 
the  character  of  renal  excretory  products.  The  course 
of  the  fluid  in  these  vessels  is  directed  by  the  valves 
which  are  placed  in  the  region  of  the  head,  and  which 
are  so  arranged  as  to  prevent  the  fluid  from  passing 
forwards  ;  the  canals  themselves  are  devoid  of  cilia, 
and,  as  in  the  Trematoda,  the  propelling  power  is  to 
be  sought  for  in  the  muscles  of  the  body  wall.  The 
fluid  is  said  to  contain  substances  which  are  chemically 
allied  to  xanthin  or  guaniii  (Sommer). 

The  simple  unsegmented  body  of  the  Rotatoria 
presents  us  with  a  correspondingly  simple  condition  of 
the  excretory  organs,  .but  their  relations  are  here 
more  easily  made  out,  owing  to  the  development  of  a 
definite  body  cavity.  There  are  several  distinct 
ciliated  and  funnel-shaped  openings  into  the  coelom, 
and  these  lead,  by  short  and  simple  canals,  into  a 
longitudinal  vessel  on  either  side ;  this  is  more  or 
less  coiled  on  its  course,  and  opens  into  the  cloaca. 
(See  page  119.) 

Similar  canals  arising  from  the  cloaca,  and  opening 
by  ciliated  infundibula  into  the  body  cavity,  are 
found  also  in  the  Oepliyrea ;  but  these  forms  are 
most  remarkable  and  interesting  for  having,  in  addi- 
tion to  these  cloacal  outgrowths,  others  which,  by 
opening  on  one  side  into  the  body  cavity  and  on  the 


252    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

other  directly  to  the  exterior,  recall  the  characters  of 
the  segmental  organs  of  the  earthworm  ;  there  may 
be  one  or  a  few  pairs  of  these  tubes,  and  their  excretory 
nature  .is  assumed  from  the  presence  in  them  of  a 
brown  concretion  (as  in  the  so-called  "  brown  tubes  " 
of  Sipunculus) ;  in  certain  forms  they  do,  without 
doubt,  lose  their  excretory,  and  take  on  the  function 
of  efferent  ducts  for  the  generative  products,  an 
arrangement  which  is  by  no  means  confined  to  the 
Gephyrea  among  animals  ;  in  Bonellia,  the  tube  which 
functions  as  the  uterus  is  developed  on  one  side  only 
of  the  body. 

It  is  of  especial  interest  to  observe  that  in  the 
developing  leech  three  pairs  of  canals  are  developed 
in  the  hinder  end  of  the  body,  and  are,  at  least,  pro- 
visional excretory  organs,  even  if  they  are  in  no  way 
related  to  the  cloacal  outgrowths  of  lower  worms. 
The  permanent  nephridia  of  the  leech  attain  to  a 
very  high  degree  of  complexity ;  it  is  possible  to 
distinguish  a  vesicle  and  a  gland,  connected  with 
one  another  by  a  vesicle  duct  (Bourne).  The  cells 
of  the  gland  are  all  penetrated  by  ductules,  and  the 
central  portion  of  each  of  its  four  constituent  lobes  is 
occupied  by  a  duct  which  opens  into  the  vesicle 
duct ;  a  plexus  of  blood-vessels  is  found  in  the 
gland,  each  cell  of  which  is  surrounded  by  a  loop  of 
that  plexus  ;  the  wall  of  the  vesicle  is  muscular,  and 
by  its  contractions  the  contents  are  expelled  to  the 
exterior.  The  marine  ecto-parasitic  leech  Pontob- 
della  is  remarkable  for  the  possession  of  a  very 
primitive  disposition  of  the  nephridia  ;  the  organ  is 
single  and  continuous,  and  consists  of  a  highly  com- 
plicated network  of  tubules ;  those  on  one  side  of  the 
body  are  continuous  with  those  of  the  other,  and  with- 
out developing  any  vesicle,  they  open  to  the  exterior 
at  regular  intervals  (Bourne). 

In  the  lower  Crustacea  an  excretory  function  is 


Chap,  vii.]  CRUSTACEA.  253 

ascribed  to  the  so-called  "shell  gland"  which  forms 
a  looped  organ  in  the  dorsal  middle  line  }  but  there 
are  as  yet  no  physiological  facts  which  confirm  this 
supposition.  In  the  higher  Crustacea,  an  organ 
which,  in  its  essential  relations,  calls  forcibly  to  mind 
the  arrangement  of  the  nephridia  of  the  earthworm, 
is  found  at  the  base  of  the  second  pair  of  antennae. 
This  is  the  so-called  "  green  gland "  of  the  cray- 
fish, where  it  presents  the  following  characters. 

An  orifice,  large  enough  to  admit  a  fairly  stout 
bristle,  leads  by  a  short  canal  into  a  wider  sac,  with 
very  delicate  walls,  which  lies  in  front  of  arid  below 
the  anterior  portion  of  the  stomach.  Below  this  wide 
thin-walled  sac  lies  a  smaller  body,  which  is  in  com- 
munication with  it  by  a  narrow  coiled  passage  ;  this 
body  consists  of  a  yellowish-brown  anterior  portion, 
which  ends  blindly,  and  of  a  green  portion,  which 
lies  between  it  and  the  duct.  The  former  is  spongy 
in  character  at  its  anterior  end,  and  the  rest  has  a 
number  of  lamelliform  processes  rising  up  from  its 
floor ;  the  green  part,  which  is  broader  and  natter, 
has  its  walls  produced  into  a  number  of  small  saccular 
outgrowths.  Ou  the  inner  surface  of  the  cells  of  this 
green  part,  and  of  the  succeeding  white  coiled  tube, 
small  highly  refractive  bodies  are  to  be  observed, 
which  are  no  doubt  of  an  excretory  nature.  The 
blood-vessels  which  bring  to  the  gland  the  materials 
that  are  to  be  excreted  by  it  arise  from  the  antennary 
and  sternal  arteries,  and  break  up  into  fine  capillaries 
in  the  walls  of  the  gland.  The  products  excreted 
are  stated  to  resemble  guanin,  but  it  will  be  understood 
that  the  small  quantities  which  can  be  collected  make 
any  chemical  investigation  a  matter  of  considerable 
difficulty; 

Wassiliew,  to  whom  we  owe  the  latest  description 
of  the  green  gland,  believes  that  three  stages  may  be 
recognised  in  the  differentiation  of  the  renal  orgari  of 


254    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Crustacea.  The  simplest  is  that  which  obtains  in 
many  Copepocla,  where  there  is  merely  a  long 
smooth  tube,  of  the  same  calibre  throughout ;  in  some 
Phyllopods  the  tube  is  enlarged  at  certain  points, 
and  more  especially  at  its  blind  end  ;  while  the  third 
and  most  complex  stage  is  that  which  obtains  in  the 
crayfish,  where  the  tube  is  widened  at  various  points, 
has  the  constituent  cells  differing  in  structure  and 
function,  and  is  folded  on  itself.  We  may  suppose 
that  the  lower  terminal  portion  is  glandular  and  ex- 
cretory, and  that  the  wide  thin-walled  sac  acts  as  a 
reservoir. 

The  organ  of  Bojanus  in  the  lamellibranch 
Mollusca  offers  many  very  striking  points  of  re- 
semblance to  the  green  gland,  but  it  differs  most  essen- 
tially in  retaining  the  primitive  character  of  having  an 
opening  into  the  body  cavity.  On  the  floor  of  the 
space  which  surrounds  the  heart  (pericardium)  we 
find,  on  either  side  of  the  ventricle,  a  small  orifice 
which  leads  into  an  elongated  chamber,  with  thick 
dark-coloured  walls,  and  narrower  at  its  hinder  than 
at  its  front  end  ;  the  walls  give  rise  to  spongy  out- 
growths, which  project  into  the  cavity,  and  which 
contain  blood  spaces,  and  are  invested  by  the  secreting 
epithelial  cells ;  at  its  hinder  and  narrower  end  this 
thick- walled  portion  opens  into  a  cavity  which  lies 
above  it,  and  which  has  thin  walls ;  this,  which 
opens  on  either  side  into  a  cloaca,  or  directly  to  the 
exterior,  is  no  doubt  the  portion  of  the  organ  which 
has  the  function  of  a  reservoir.  Here,  then,  we  have 
again  an  arrangement  which  may  be  explained  as 
that  of  a  tube,  folded  on  itself,  and  having  part 
differentiated  into  a  glandular  secreting  region  and 
part  into  a  collecting  region  or  reservoir.  The  gland 
is  said  to  secrete  uric  acid. 

In  the  fresh-water  mussel  the  products  of  the  gene- 
rative glands  pass  to  the  exterior  qiiite  independently 


chap,  vii.i  MOLLUSC  A.     •  255 

of  the  ducts  of  the  renal  organ,  but  in  others  the  latter 
are  used  as  a  means  of  passage  for  the  genital 
products,  just  as  are  the  brown  tubes  in  the  Ge- 
phyrea.  In  Spondylus  the  products  are'  discharged 
into  the  renal  cavity  ;  in  Mytilus  (the  sea  mussel) 
there  is  a  distinct  genital  duct,  which  opens,  however, 
on  the  same  papilla  as  the  renal ;  while  in  Anodon 
and  others  the  two  ducts  are  completely  separated. 
This  use  of.  the  renal  ducts  by  the  generative  glands 
is  regarded  by  Hubrecht  as  a  more  primitive  arrange- 
ment, but  it  was,  he  thinks,  preceded  by  one  in 
which  the  genital  products  first  escaped  into  the  peri- 
cardium, whence  they  were  taken  up  by  the  renal 
organ. 

In  the  lower  Cephalophora  the  renal  glands  are 
paired,  and  either  open  separately,  as  in  Dentalium,  or, 
as  in  Proneomenia,  the  ducts  unite  posteriorly  ;  in  the 
more  differentiated  Gastropoda  we  find  that  the 
organ  of  one  or  other  side  is  affected  by  that  torsion 
of  the  body,  which  has  so  pronounced  an  influence  on  the 
development  of  all  the  other  organs  of  these  molluscs. 
In  the  Pulmonata  the  external  orifice  is  obscured 
by  opening  into  the  cavity  of  the  air  chamber,  but  as 
this  is  merely  formed  by  the  folding  over  and  attach- 
ment of  one  edge  of  the  mantle,  there  is  no  reason  to 
suppose  that  there  is  any  real  change  in  its  essential 
morphological  characters.  Sometimes  the  terminal 
portion  of  the  gland  has  muscular  tissue  developed  in 
its  walls,  and  in  some  Heteropods  and  Pteropods  the 
whole  organ  is  capable  of  contracting. 

In  the  Cephalopoda  there  are  either  one 
(dibranchiata)  or  two  (tetrabranchiata)  pairs  of  renal 
organs  (Fig.  105  ;  rr).  They  are  richly  supplied  with 
blood-vessels,  which  enter  into  the  lamelliform  pro- 
cesses that  project  into  their  interior ;  they  open  by 
a  somewhat  circuitous  course  into  that  portion  of 
the  body  cavity  which  surrounds  the  heart,  and 


256    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


communicate  by  ducts,  or  ureters,  with  the  exterior. 
The  chief  product  of  these  excretory  glands  is  stated 
to  be  phosphate  of  calcium. 

Among  the  air-breathing  Arthropods  we  find 
that  the  excretory  organs  are  outgrowths  of  the 
terminal  portion  of  the  intestine,  which,  varying 
more  or  less  in  size  and  number,  extend  some  way 

into  the  body  cavity ; 
they  are  the  organs  that 
are  known  as  the  Mal- 
ptgliiaii  tubes,  and 
uric  acid  has  been  repeat- 
edly proved  to  be  found 
in  them.  It  is  possible, 
but  it  is  by  no  means 
certain,  that  they  are  ho- 
mologous with  the  rectal 
excretory  organs  which 
we  have  already  found 
in  the  Gephyrea  and  the 
Rotatoria.  They  may  be 
completely  wanting,  as  in 
many  of  the  Aptera  and 
Pycnogonida  ;  there  may 
be  two  as  in  the  harvest- 
men  (Opilionida)  where 
they  are  considerably 
branched  ;  or  four,  as  in  the  blowfly,  where  they  are 
very  short;  or  six,  as  in  the  greater  number  of  insects. 
Sometimes  there  is  a  much  larger  number,  the  cock- 
roach having  from  twenty  to  thirty,  and  some  Hymen- 
optera  as  many  as  one  hundred  and  fifty.  They  are 
sometimes  arranged  in  bundles,  and  where  there  is  a 
common  duct  leading  to  the  exterior,  its  walls  are 
sometimes  provided  with  muscular  tissue,  which  aids 
in  the  expulsion  of  the  contents. 

The  Malpighian  tubules  are  often  of  great  size  in 


Tig.   105.— Eespiratory  and  Eejal 
Organs  of  Septa. 

a.  Aorta ;  v,  vena  cava ;  v*,  posterior 
vcnas  cavas,  r,  heart ;  d,  enlargements 
of  the  veins  (auricles) ;  e,  branchial 
hearts  ;  6  6,  gills  ;  r  r,  kidneys. 


Chap,  vii.]  RENAL  ORGANS  OF  CHORD  ATA.  257 

the  larvae  of  insects,  and  a  large  quantity  of  renal  ex- 
cretion is  collected  in  the  rectum  during  the  pupal 
stage.  This  phenomenon  may,  as  Gegenbaur  has 
pointed  out,  be  well  brought  into  relation  with  the  fact 
that  it  is  at  this  stage  that  the  "  most  intense  plastic 
activity  is  going  on  in  the  organism  in  con- 
nection with  the  development  of  the  perfect  body." 
The  blowfly,  when -it  first  emerges  from  the  pupa 
case,  excretes  a.  semi-solid  mass  of  nearly  pure 
uric  acid  (Lowne). 

Peripatus  is  remarkable  for  the  possession  of 
organs  which  have  a  general  resemblance  to  the  seg- 
mental  organs  or  nephridia  of  the  earthworm  and  other 
Annulata,  and  are  like  them  found  in  all  the  segments 
of  the  body,  but  those  in  the  three  foremost  pairs  of 
tegs  are  very  rudimentary.  A  typical  nephridinm 
opens  at  the  base  of  each  leg  ;  the  tube  leading  to  the 
opening  is  narrow,  but  is  continued  internally  into  a 
large  sac,  which  appears  to  act  as  a  bladder  or  collect- 
ing organ  ;  this  sac  is  continuous  with  a  coiled  tube, 
which  opens  by  a  funnel-shaped  orifice  into  the  cavity 
of  the  body. 

No  definite  information  has  been  acquired  as  to  the 
possession  of  a  renal  gland  by  Amphioxus.  The 
Urochordata  are  remarkable  for  the  fact  that  the 
uric  acid  secreted  from  their  blood  is  not  carried  away 
to  the  exterior,  but  collected  into  spherical  vesicles 
of  large  size,  which  lie  in  a  mass  round  the  intestine  ; 
in  Lithonephrya  the  cavity  of  the  renal  organ  is 
almost  filled  by  a  single  large  concretion ;  in  other 
Molgulidse,  where  the  presence  of  uric  acid  has  been 
definitely  proved  by  the  colour  reactions  given  with 
nitric  acid  and  with  ammonia  ("  murexide  test  "),  the 
renal  organ  has  the  form  of  a  sac,  which  lies  close  to 
the  pericardium. 

In  the  Verteforata  we  find  that,  with   a  general 
resemblance  to  the  nephridia  of  the  ringed  worms,  the 
R— 16 


258  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

excretory  organs  present  some  very  complex  characters 
as  we  ascend  the  series.  In  the  examination  of  these 
organs  it  will  be  found  convenient  to  make  use  of  cer- 
tain technical  terms. 

The  Pronephros  (or  head-kidney)  is  a  small 
glandular  body,  with  one  or  more  funnel-shaped 
ciliated  openings  into  the  body  cavity ;  it  is  ordinarily 
placed  far  forwards,  and  is  provided  with  a  duct,  the 
so-called  segmental  duct.  Like  many  other  renal 
organs,  it  is  provided  with  a  special  blood-supply  in  the 
shape  of  a  coil  of  vessels,  or  glonierulus. 

The  mesonephros  (Wolfftan  body)  consists  of 
a  series  of  glandular  tubes  which  open  by  funnel- 
shaped  openings  into  the  body  cavity,  and  pour  their 
secretion  into  the  common  segmental  duct. 

The  metanephros  (kidney  of  Amniota)  con- 
sists of  a  complex  of  coiled  tubes  which  open  into  a 
special  duct,  which  is  derived  from  that  of  the 
mesonephros. 

All  these  three  organs  may  be  developed  in  one 
and  the  same  individual,  but  they  are  not,  in  higher 
forms,  in  active  function  at  the  same  time ;  the 
metanephros  is  developed  in  the  Amniota  only,  though 
an  indication  of  its  existence  is  to  be  seen  in  Elasmo- 
branchs. 

In  the  adult  Cyclostoniata  the  mesonephros  is 
found  in  its  simplest  conditioL,  for  it  there  consists  of 
a  segmental  duct  with  tubes  (Fig.  106;  a,  b)  given  off  on 
one  side,  and  ending  in  a  blind  enlargement ;  in  this 
enlargement  an  artery  (d)  breaks  up  into  a  plexus  of 
fine  vessels  which  form  the  glomerulus  (c),  and  thence 
the  blood  passes  into  the  efferent  artery  (e).  An- 
teriorly to  this  there  is,  in  Myxine,  a  pronephros, 
which  disappears  in  the  adult  Petromyzon,  where  the 
whole  kidney  is  more  compact. 

In  the  Elasmotoranchii  the  segmental  tubes  in 
the  hinder  part  of  the  mesonephros  unite  with  one 


Chap.  VII.] 


FISHES;  AMPHIBIA. 


259 


another  before  they  open  into  the  common  efferent 
duct,  and  the  pronephros  would  appear  to  be  absent 
even  at  the  very  earliest  stages.  In  them,  as  in  Fishes 
generally,  the  renal  organs  are  of  great  length,  as  com- 
pared with  those  of  the  higher  Yertebrata. 

In  the  sturgeon,  among  the  Oanoids,  the  kidneys 
extend  from  just  behind  the  skull  as  far  as  the  cloaca, 
and  differ  in  width  in  different  regions ;  in  them,  and 


A  I  B 

Fig.  106.—  Mesonephros  of  Bdellostoma. 

A.  ff.,Segmental  duct ;  6,  segniental  tube  ;  c,  glomeruHie.  B.  A  part  more  highly 
magnified,  showing  one  duct  with  its  afferent  vessel  d,  and  its  efferent  e. 
(After  J.  Mill  lef.) 

in  the  Teleostei,  there  is  a  great  reduction  in  the 
number  of  separate  ductules  which  pass  from  the  sub- 
stance of  the  kidney  into  the  efferent  duct. 

In  the  Urodela  the  mesonephros  is  of  considerable 
length,  and  gives  off  a  number  of  ducts  (Fig.  107)  ;  in 
the  ffogy  which  may  be  taken  as  a  type  of  the  Aimra, 
the  kidney  is  much  shorter,  and  the  efferent  duct 
(ureter)  is  closely  applied  to  the  lower  third  of  the 
kidney ;  if,  however,  we  make  an  examination  of  a 
longitudinal  section  of  the  kidney  under  a  low  magni- 
fying power,  we  shall  see  that  the  substance  of  that 


260  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


organ  consists  of  a  number  of  delicate  and  convoluted 
tubes,  which,  when  mapped  out  in  diagram,  have 
much  the  appearance  shown 
in  the  figure  (Fig.  109;  A). 

Just  as  in  nearly  all  Ich- 
thyopsida  the  pronephros  is 
seen  to  be  a  purely  larval 
organ  (Balfour),  so  in  the  Am- 
niota  the  mesonephros  makes 
way  for  the  metanephros,which 
is  here  preceded  for  a  very 
short  time  by  the  pronephros. 
The  metanephros  appears  to 
be  a  further  development  of 
the  hinder  part  of  the  mesone- 
phros, and,  like  it,  it  retains 
throughout  life  evidence  of 
being  composed  of  a  system  of 
tubules,  which  advance  in  com- 
plexity as  we  rise  in  the  series. 
The  truth  of  this  will  be 
shown  by  a  study  of  the  mi- 
nute anatomy  of  the  kidney 
of  a  tortoise  (Fig.  109;  B),  and 
a  comparison  of  it  with  that 
of  a  pigeon  (Fig.  109;  c),  and 
of  a  man  (D). 

So,  again,  with  advancing 
complexity  of   internal  struc- 
Fig.  107.  —  Diagram  of  the   ture,   we  observe,   as  we  pass 

JMe.°neH0rOS  t°efsti?.  ^    fr°m      RePtiles     to     Birds      or 

kidney.    ('Modified  'from   Mammals,  that  the  length  of 
Sprengei.)  the    kidney    diminishes,    and 

that  it  becomes  limited  to  the  lumbar  region  of  the 
body,  while  the  ducts  that  open  into  the  pelvis  of  the 
kidney  are  reduced  in  number  and  increased  in  size. 
The  more  important  macroscopic  differences  in  the 


Chap,  vii.]       SAUROPSIDA  ;  MAMMALIA. 


261 


kidneys  of  the  Ainniota  obtain  in  the  relative  position 
of  the  two  organs ;  in  Snakes,  for  example,  not  only 
are  the  kidneys  elongated  in  relation  to  the  general 

form  of  the  body,  but  ^    . 

(rAo 

TK 


one  lies  a  good  deal 
in  front  of  the  other ; 
this  difference  of 
level  between  the 
two  kidneys,  which 
is  clearly  an  arrange- 
ment for  more  con- 
venient packing,  may 
be  seen  also  in  some 
Mammals  (rabbit)  ; 
secondly,  the  kidneys 
vary  considerably  in 
their  external  form  ; 
thus,  those  of  the 
lizard  are  only 
slightly,  those  of  the 
Ophidia  much  more, 
broken  up  into  lobes; 
this  is  a  difference, 
also,  which  obtains 
between  young  and 
old  forms,  for  in  the 
latter  the  number  of 
lobes  is  much  greater 
than  in  the  former. 


FK 


lOe.-Urogenit^ljLpparatus  of  a  Male 

N  Kf  Kidneys  :  urur,  ureters  :  +  their  point  of 
origin  ;  8  s',  their  opening  into  the  cloaca  cl, 
H  o,  testes;  FK,  fat  body; 


vena  cava    inferior  ; 
tAfter  Wiedersheim.) 


AO,  aorta;  cv. 
efferent    veins. 


fif     i™4-^     4-1^0.      c.r  nnaa 
tit     mtO     the      Spaces 

between    the    trans- 

verse processes  of  the  vertebrae  of  the  pelvic  region, 
The  lobes  of  the  kidney,  after  appearing,  do,  in  many 
Mammals,  again  fuse  with  one  another,  so  that 
while  the  Cetacea,  in  which  this  process  does  not 
obtain,  may  have  as  many  as  two  hundred  lobes,  the 


262  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

outer  surface  of  the  kidney  of  a  rabbit  or  of  a  man  is 
quite  smooth ;  this  fusion  affects  also  the  inner  sub- 
stances of  the  lobes,  and  in  man  three,  or  only  two, 
tubes  open  directly  into  the  pelvis  of  the  organ. 


Fig.  109.— Diagrams  of  the  Urinary  Tubules  of 

A,  The  frog;  B,  tortoise;  c,  pigeon  (after  Hufner);  D,  man  (after  Ludwig) ;  I., 
glomerulus ;  u.  to  vii.,  various  regions  of  the  tubule. 

The  duct  that  carries  to  the  exterior  the  secretion 
of  the  kidney  is  ordinarily  known  as  the  ureter ; 
so  long  as  the  pronephros  persists,  the  segmental 
duct  and  the  ureter  are  one  and  the  same  ;  when 
the  mesonephros  appears,  the  duct  becomes  divided 
into  two,  the  Miilleriaii  duct  (which  has  no  con- 
nection with  the  kidney,  but  forms  the  oviduct  of  the 


chap,  vii.]        URETERS  AND  BLADDERS.  263 

female,  and  undergoes  more  or  less  degeneration  in 
the  male),  and  the  Wolffiaii  duct,  which  remains 
connected  with  the  mesonephros,  and  carries  away  its 
products  and  those  of  the  testes  in  the  male.  The 
ureter,  like  the  collecting  ducts  of  the  inetanephros, 
becomes  developed  from  part  of  the  Wolffian  duct, 
which,  when  this  ureter  is  'present,  only  carries  away 
the  secretion  of  the  testes. 

In  the  Cyclostomata,  the  ducts  that  have  the 
functions  of  ureters  open  to  the  exterior  by  a  papilla 
(the  urinogenital  papilla),  which  is  placed  .behind  the 
anus ;  they  first,  however,  open  into  a  cloaca,  into 
which  also  the  generative  products  make  their  way  by 
the  abdominal  pores.  Among  the  Ichthyopsida 
the  renal  and  generative  products  ordinarily  pass  into 
a  cloaca,  which  is  common  to  them  and  to  the  rectal 
orifice  of  the  intestine ;  but  in  the  Teleostei  the 
genito-urmary  is  distinct  from  and  posterior  to  the 
rectal,  and  the  urinary  pore  is,  as  a  rule,  separate 
from  and  behind  the  genital.  The  terminal  portion 
of  the  ureteric  ducts  of  fishes  is  often  enlarged,  to 
form  a  so-called  bladder ;  this,  however,  must  not  be 
regarded  as  the  homologue  of  that  of  Amphibians, 
or  of  the  Amniota,  where  the  bladder  is  an  out- 
growth of  the  ventral  wall  of  the  cloaca.  In  the 
Amphibia  and  in  some  Reptilia  this  bladder  retains 
its  primitive  position,  or,  in  other  words,  does  not 
become  part  of  the  direct  line  of  passage  between  the 
kidneys  and  the  exterior ;  in  the  Ophidia,  Crocodilia, 
and  Aves,  the  bladder  is  atrophied. 

It  is  in  the  Mammalia  only  that  the  bladder  is 
found  on  the  direct  line  of  passage  between  the  ure- 
ters and  the  exterior,  and  is  not  so  found  in  the  lowest 
division  or  Prototheria,  where  rather  it  occupies 
the  same  position  as  in  the  frog  ;  in  the  Metatheria 
the  ureteric  ducts  open  into  the  base  of  the  bladder ; 
in  the  Eiitheria  they  open  at  various  points  along  its 


264  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Fig.  110.  —  Renal  and  Generative  Organs  of 

Ericulus  sct(j8us.  , 


course  ;  thus  in 
Gymnura  they 
open  into,  and  in 
Ericulus  near,  the 
neck  (Fig.  110); 
in  in  an  th  ey  e  n  te  r 
the  walls  of  the 
bladder  at  its 
base,  but  run  in 
its  walls  for 
about  three- 
quarters  of  an 
inch  before  open 
ing  into  the  ca- 
vity of  the  blad- 
der ;  in  the  rab- 
bit they  open  far 
up  on  the  hind 
wall,  and  in  the 
coney  at  the  top. 
Thebladderitself 
varies  consider- 
ably in  size  and 
form  ;  when,  as 
in  the  higher 
forms,  the  cloaca 
disappears,  the 
urino-genital  ori- 
fice is  in  front  of 
the  anus, 
page  170.) 


Kidneys  ;  /,  testes  ; 
ut,  ureter ;  ?-'d,  vas 
deferens ;  6, bladder; 
pg,  prostate  gland ; 
eg,  Cowper's  gland ; 
lpt  levator  penis;  cc, 
cystic  urethra;  pr. 
prepuce,  divided  and 
reflected ;  p,  penis. 
lAf  ter  Dobson.) 


Chap,  viii.]  SPECIAL  SECRETIONS.  265 

The  two  important  constituents  of  the  urine  of  the 
Vertebrata  are  urea  and  uric  acid ;  the  former  is 
the  preponderating  constituent  in  the  Mammalia, 
the  latter  in  the  Sauropsida,  and,  as  urea  is  readily 
soluble  in  water,  while  uric  acid  is  very  insoluble,  we 
find  that  the  renal  products  of  the  Sauropsida  are  or- 
dinarily semi-fluid,  and  dry  rapidly  on  exposure  to 
the  air.  The  urine  of  carnivorous  mammals  is  more 
concentrated  and  more  acid  than  that  of  man ;  that 
of  herbivorous  forms  is  ordinarily  alkaline,  but  when 
it  is  acid  in  reaction,  uric  acid  is  as  abundant  as 
in  the  lion  or  the  tiger  (Garrod) ;  the  herbivora  have 
a  large  quantity  of  hippuric  acid,  which  is  only  found 
in  small  quantities  in  man. 


CHAPTER    VIII. 

ORGANS    OF    SPECIAL    SECRETIONS. 

IN  addition  to  the  various  secretions,  such  as  saliva 
and  bile,  or  excretions  such  as  the  uric,  there  are 
others  which,  though  dependent,  of  course,  on  the 
activity  of  protoplasmic  cells,  are  special  and  peculiar 
to  different  animals,  and  are  not  a  necessary  result  of 
protoplasmic  activity ;  such,  for  example,  is  the  poison 
of  the  scorpion,  or  the  ink  of  the  cuttlefish. 

Poison  or  venom  glands.  —  While  in  the 
Ophidia  or  the  mad  clog  the  poison  is  due  to  a  modifi- 
cation of  the  proper  salivary  glands,  we  find  special 
glands  developed  in  various  Arthropods.  Among  the 
Arachnida,  the  spider  is  provided  with  tubular 
glands  placed  at  the  base  of  the.  chelicerse,  or  first  pair 
of  appendages,  which  open  by  a  narrow  duct  at  the 
orifice  at  the  end  of  these  organs ;  the  two  last  joints 
are  movable  on  one  another,  and  are  thus  enabled  to 


266  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY.- 

bite  their  victim  before  injecting  the  poison  ;  though, 
as  in  the  case  of  the  Tarantula,  the  ill  effects  of  their 
venom  have  been  somewhat  exaggerated,  there  is  no 
doubt  that  the  poison  of  many  spiders  is  capable  of 
inflicting  mortal  injuries;  the  statement  that  the 
West  Indian  My  gale  avicularia  is  able  to  catch  and 
kill  small  birds  appears  to  be  true.  In  the  Scorpions 
the  poison  glands,  which  are  oval  in  form,  and  have 
an  outer  layer  of  muscular  tissue,  are  situated  in 
the  terminal  segment  of  the  body  ;  their  ducts  open 
at  the  tip  of  the  spinous  process  at  the  end  of  the  tail, 
which  is  recurved  when  the  animal  strikes  a  blow. 
Among  insects,  many  of  the  Hymenoptera  are  pro- 
vided with  racemose  organs  placed  in  the  hinder  part 
of  the  body,  which  secrete  a  fluid,  the  irritating 
effects  of  which  appear  to  be  due  to  the  contained 
formic  acid  ;  the  venom  is  injected  by  a  sting,  which 
consists  of  a  median  piece  grooved  longitudinally,  and 
of  two  side  pieces  which,  on  becoming  closely  applied 
to  it,  convert  the  groove  into  a  capillary  canal  along 
which  the  fluid  flows.  The  venom  is  not  always  used 
merely  as  a  means  of  offence,  many  Hymenoptera 
stinging  other  insects  for  the  purpose  of  paralysing 
them  while  they  carry  them  to  their  young,  to  which 
they  will  serve  as  food. 

Various  Fishes  are  provided  with  defensive  organs 
possessing  venomous  properties  ;  such  are  the  dorsal 
spines  of  the  weavers,  which  are  deeply  grooved  and 
charged  with  fluid  mucus.  In  Synanceia  the  free  half 
of  each  dorsal  spine  bears  a  pear-shaped  bag  in  which 
the  milky  poison  is  contained.  In  Thalassophryne, 
from  Panama,  the  sac  is  placed  at  the  base  of  the 
spine,  and  as  it  is  without  any  muscular  sheath,  the 
poison  can  only  be  ejected  by  the  pressure  exerted  on 
the  sac  when  the  spine  enters  the  body  (Giinther). 
The  integument  of  many  Amphibia  is  richly  pro- 
vided with  glands,  which  secrete  a  viscid  fluid  possessed 


Chap,  vni.]  SILK  ORGANS.  267 

of  more  or  less  well-marked  irritating  properties ;  a 
familiar  example  of  this  is  the  common  toad,  the 
handling  of  which  is  often  succeeded  by  inflamma- 
tion of  the  eyelid.  Experiments  with  subcutaneous 
injections  of  the  dermal  secretion  of  the  Triton  show 
that  it  appears  to  have  an  effect  on  the  heart,  and 
that  of  the  salamander  on  the  central  nervous  system. 

Silk  organs.- The  result  of  the  secretion  of  the 
silk  organs  of  Spiders  is  the  well-known  web  ;  but 
the  secreted  product,  when  it  first  appears,  is  a  vis- 
cous transparent  liquid,  which  rapidly  hardens  on  ex- 
posure to  the  air,  and  then  forms  threads.  The  silk  is 
produced  in  various  glands,  which,  however  different 
in  form,  are  always  found  distributed  among  the  con- 
tents of  the  abdomen  ;  the  secretion  makes  its  way 
to  the  exterior  through  the  so-called  "spinnerets,"  of 
which  there  are  ordinarily  three  pairs  ;  these  have  the 
form  of  obtusely  conical  papillae,  the  tips  of  which  are 
provided  with  a  number  of  pores  through  which  the 
silk  escapes  to  the  exterior.  This  silk  is  used  in  very 
various  ways  ;  some  spiders  make  cells  or  tubes  for 
themselves,  some  scatter  the  threads  about,  with  the 
obvious  object  of  entangling  an  approaching  prey, 
while  many  make  nets  for  the  purpose  of  entrap] ring 
victims.  The  so-called  mason,  or  trap-door  spiders, 
spin  a  number  of  successive  webs,  which  unite  to  form 
a  door  for  the  pit  in  which  they  dwell.  Clotho  makes 
a  net-like  tent,  in  which  the  young  are  concealed.  In 
many  cases  the  webs  are  spun  with  considerable 
rapidity,  the  common  English  spiders  being  able  to 
make  one  in  about  an  hour. 

Among  the  Iiisecta,  silk-producing  glands  are 
best  seen  in  the  larvaB  of  the  Lepidoptera,  where  they 
have  the  form  of  two  long  caecal  tubes,  placed  one  on 
either  side  of  the  intestine,  and  opening  by  narrow 
ducts  at  the  base  of  a  spinneret,  which  is  developed 
on  the  labium.  As  in  the  spider,  the  silk  is  at  first 


268  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

fluid,  but  soon  hardens  on  exposure  to  the  air.  The 
silk  thus  secreted  may  be  used  as  a  kind  of  attaching 
rope,  as  in  some  moths  (Tortrix),  or  it  may  form  an 
investment  for  the  larva,  as  is  the  case  with  the  silk- 
worm. In  the  larvae  of  the  ant-lion  (Myrmeleoii)  the 
silk  is  secreted  by  the  rectum,  and  escapes  by  a  spin- 
neret which  is  placed  near  the  anus. 

A  secretion  of  somewhat  similar  character  is  made 
by  some  of  the  JLamellibranchiata,  where  the 
foot  secretes  a  soft  substance,  which  becomes  hard  and 
chitinous  on  exposure  to  the  air ;  this  "  foyssus " 
may  consist  of  threads  fine  enough  to  be  woven  into 
gloves  (Pinna),  or  of  coarser  filaments,  as  in  the  sea 
mussel,  or  they  may  form  firmer  chitinous  plates. 
The  function  of  these  byssal  threads,  as  may  be  well 
seen  in  the  Glochidia,  or  young  of  the  fresh- water 
mussel,  is  one  of  attachment. 

Offensive  organs  of  a  somewhat  similar  character 
are  to  be  found  in  certain  Holothurians.  Con- 
nected with  and  opening  into  the  cloaca  are  a  number 
of  tubes,  compacted  together  into  a  more  or  less  large 
mass,  and  occupying  in  some  cases  a  considerable 
portion  of  the  body  cavity.  The  secretion  of  these 
Cuvierian  organs  is  expelled,  on  irritation,  in  the 
form  of  fine  tubes,  which  are  capable  of  considerable 
extension,  and  which  also  swell  up  in  the  water. 
These  expelled  threads  have  a  remarkable  power  of 
adhering  to  any  object  which  they  may  touch,  and  of 
more  or  less  completely  entangling  it  and  preventing 
its  escape.  An  English  example  of  a  Holothurian 
thus  provided  is  afforded  by  the  so-called  "  Cotton- 
spinner  "  (Holothuria  nigra)  ;  the  tubes  are  known  to 
have  an  irritating  effect  on  the  human  skin. 

True  electric  organs  are  developed  in  Torpedo 
and  other  rays,  in  the  eel  (Gymnotus),  and  in  the 
teleostean  Malapterurus.  They  are  either  placed  in 
the  head  (Torpedo),  or  in  the  tail  (Gymnotus),  or  over 


Chap.  VllLlELECTRICAA'DPlfOSPORESCENTORGANS.  269 

the  whole  surface  of  the  body  (Malapterurus).  They 
are  very  richly  supplied  with  nerves,  and  appear  to  be 
modifications  of  muscular  tissue,  which  they  so  far 
resemble  in  physiological  activity  that  they  are  under 
the  control  of  the  fish  ;  are  exhausted  after  a  certain 
period  of  activity ;  and  are  brought  into  a  tetanic 
condition  in  which  a  number  of  discharges  succeed 
one  another  involuntarily,  when  their  possessor  is 
treated  with  strychnine.  In  the  Torpedo  the  organs 
are  made  up  of  a  number  of  hexagonal  bodies,  each  of 
which  is  divided  into  a  number  of  cells  by  intervening 
septa,  between  which  is  a  clear  gelatinous  fluid,  or 
mucous  tissue ;  the  Torpedo  has  about  a  thousand 
electric  prisms,  and  Gymnotus  is  said  to  have  two 
hundred  and  forty  electric  cells  in  one  inch  of  its 
electric  organ.  Though  the  effect  of  these  bodies  has 
no  doubt  been  exaggerated  by  travellers,  it  is  clear 
that  they  are  capable  of  producing  sufficiently  remark- 
able results. 

Curiously  allied  in  the  details  of  their  structure  to 
the  organs  just  mentioned  are  the  so-called  eye-like 
spots  found  in  various  fishes  (Argyropelecus,  etc.), 
and  best  developed,  apparently,  in  deep-dwelling 
forms.  The  special  activity  of  these  organs  does 
not,  however,  exhibit  itself  in  the  production  of 
electricity,  but  of  light ;  they  are  phosphorescent 
organs.  Kolliker,  more  than  a  quarter  of  a  century 
ago,  suggested  that  the  luminous  organs  of  insects, 
such  as  the  Lampyridse  and  Elateridae,  were  allied  to 
the  electric  organs  of  fishes.  So  far,  however,  as  we 
know  anything  as  to  the  mode  of  activity  of  these 
bodies,  which  are  richly  supplied  with  tracheae,  and 
appear  to  vary  in  brightness  with  the  movements  of 
expiration  and  inspiration,  we  are  led  to  suppose  that 
the  oxygen  taken  in  from  the  air  is  a  factor  of  con- 
siderable importance. 

Phosphorescence  is  exhibited  by  such  simple 


270  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

forms  as  Noctiluca  among  the  Protozoa,  by  many 
Medusae,  by  the  Pennatulidae,  by  Beroe  and  Cestus ; 
among  the  Annulata,  it  has  been  observed  in  the 
earthworm,  where  it  appears  to  have  its  seat  in  the 
clitellum,  and  in  various  marine  Polychseta ;  in 
Polynoe  the  light  is  of  a  greenish  colour ;  in  Poly- 
cirrus  pale-blue  ;  among  the  Tunicata,  Doliolum  has 
been  observed  to  be  phosphorescent ;  and  the  com- 
pound ascidian.  Pyrosoma  is,  as  its  name  implies, 
remarkably  so.  As  these  animals  float  in  great 
companies,  they  have  been  spoken  of  as  a  "  shoal  of 
miniature  pillars  of  fire  gleaming  out  of  the  dark  sea, 
with  an  ever-waning,  ever-brightening,  soft  bluish 
light "  (Huxley), 

The  physiology  of  phosphorescence  is  incompletely 
known.  Panceri  observed  in  Pemiatula  that  the 
activity  was  exhibited  only  by  the  eight  longitudinal 
bands  of  fatty  substance  placed  on  the  outer  wall  of 
the  stomach ;  and  these  bands  are  luminous  after 
removal  from  the  body.  They  can  be  set  in  activity 
by  various  stimuli,  mechanical,  chemical,  and  so  on. 
When  exactly  studied  by  electrical  stimuli,  there  is 
found  to  be  a  latent  period  of  |ths  of  a  second.  The 
fact  that  many  deep-sea  forms  are  coloured  points  to 
the  existence  of  light  in  great  abysses  of  the  ocean  ; 
this  can  only  be  due  to  phosphorescent  animals,  as  we 
cannot  accept  the  supposition  that  sunlight  can  pene- 
trate to  any  considerable  depth. 

The  observations  of  Aubert  and  Dubois  on  Pyro- 
phorus,  one  of  the  well-known  phosphorescent  beetles 
(Elateridse),  have  revealed  the  remarkable  fact  that 
the  most  persistent  of  the  rays  of  light  are  the  green, 
and  that,  with  increasing  brightness,  the  last  rays  to 
appear  are  those  that  are  least  refractive,  whereas,  as 
a  rule,  they  are  the  first  to  be  seen.  This  light  has 
been  observed  to  have  an  action  on  sensitised  paper. 

The    colours    of    animals    are    due    either    to 


Chap,  viii.]  PIGMENTS.  271 

pignments,  which  are  formed  by  protoplasmic  cells, 
or  to  the  minute  structure  of  the  surfaces  of  parts 
of  their  body,  which  .variously  affect  the  rays  of 
which  white  light  is  composed ;  or  to  these  two 
causes  combined.  Although,  in  most  cases,  the 
pigment  is  superficial,  it  is  not  always  so ;  thus,  the 
11  colour "  of  a  man's  cheek  is  due  to  the  haemo- 
globin in  the  blood,  as  is  shown  by  the  yellow 
colour  of  those  affected  with  jaundice,  in  which 
disease  haemoglobin  is  converted  into  bile  pigment ; 
or  the  staining  of  the  skin  in  syphilis,  the  poison  of 
which  seems  to  be  particularly  destructive  of  the  red 
blood  corpuscles.  Similarly,  the  red  eye  of  an  albino 
is  due  to  the  absence  of  pigment  in  the  iris  and  the 
retina,  in  consequence  of  which  the  red  blood  is 
seen  through  the  transparent  tissues  of  the  eye  ;  when 
the  retina  is  pigmented,  but  the  iris  free  of  pigment, 
the  red  colour  of  the  blood  is,  by  interference,  given 
a  blue  shade,  and  the  eye  is  said  to  be  blue.  When 
pigment  is  laid  down  also  in  the  iris,  the  red  colour  is 
more  or  less  completely  obscured,  and  we  get  light- 
brown  or  dark-brown  eyes. 

No  distinctive  white  pigment  has  yet  been  de- 
tected, and  the  whiteness  of  certain  animals  must  be 
explained  as  caused  by  the  presence  of  air-cells  or 
spaces  in  which  none  of  the  impinging  light  is  ab- 
sorbed. Many  of  the  colours  seen  in  animals  are  due 
to  the  admixture  of  different  pigments ;  red  overlaid 
by  yellow  giving,  for  example,  orange,  or,  when  thinly 
spread  out,  pink  of  various  shades,  proportionate  to 
the  amount  of  colouring  matter  present. 

Many  colouring  matters  are  soluble  in  alcohol,  and 
not  a  few  are  fluorescent ;  in  some  cases  they  have 
been  observed  to  present  absorption  bands  when 
examined  with  the  spectroscope,  and  these  bands  are 
definite  and  characteristic  of  the  pigment.  Among 
the  Protozoa  a  blue  colouring  matter  has  been  observed 


272  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

in  Stentor  (stentoriii) ;  in  some  corals  and  hydroids 
there  is  a  red  pigment  (polyperythrin) ;  ctiloro- 

cniorin  has  been  obtained  from  various  Poly cb  seta  ; 
pent acriai hi  from  Pentacrimis  ;  and  antedoniii 

from  Antedon  and  a  Holothurian  ;  the  terms  cms- 
taceorubrin,  aplysiopurpurin,  ianthiiiin, 
and  bonellein  explain  themselves.  Zooxanthin, 
zooerythrin,  zoofulvin,  and  turacin  have  been 
extracted  from  the  feathers  of  various  birds. 

The  question  whether  the  characteristic  colouring 
matter  of  plants  (chlorophyll)  is  formed  by  animals 
is  complicated  by  the  undoubted  fact  that  a  number 
of  lower  organisms  have  associated  with  them  green 
algae,  which  are  not  so  much  parasitic  as  symbiotic, 
inasmuch  as  the  oxygen  which  they  evolve  in  the 
presence  of  sunlight  is  of  advantage  to  the  anima1 
with  which  they  live  ;  such  are  the  so-called  yellow 
cells  of  Anthozoa  and  Radiolaria.  Where  no  cell- 
nucleus  is  seen  to  be  associated  with  the  green  cor- 
puscles, as  is  the  case  in  Spongilla  and  Hydra,  w« 
have  no  reason  for  refusing  to  suppose  that  the 
chlorophyll  has  been  formed  by  the  animal  itself. 

Some  animals  possess  the  power  of  changing  more 
or  less  rapidly  in  colour  ;  as,  for  example,  the  cuttle- 
fish or  the  chameleon.  This  property  is  due  to  the 
presence  of  chromatophores,  or  aggregations  of 
pigment  surrounded  by  an  envelope ;  the  latter  is 
provided  with  radiating  muscles,  by  the  contraction 
or  expansion  of  which  the  chromatophore  becomes 
flattened  out,  and  the  contained  pigment  displayed  or 
drawn  into  a  denser  mass,  so  as  to  appear  merely  as  a 
dark  spot.  In  the  chameleon,  where  the  play  of 
colour  is  not  so  rapidly  effected  as  in  the  cephalopod, 
there  are  no  radial  fibres.  Similar  structures  are  found 
less  well  developed  in  other  lizards,  and  in  some  fishes*. 

The  effects  of  structure  are  best  shown  by  what  are 
ordinarily  known  as  metallic  colours.  These  are 


chap,  viii.]          METALLIC  COLOURS.  273 

well  seen  in  the  wings  of  various  insects,  the  scales  of 
which  are  marked  by  extremely  delicate  strise,  or  covered 
by  a  thin  membrane.  The  rays  of  white  light  suffering 
interference  are  broken  up  into  their  constituent  parts, 
and  different  colours  are  produced  in  different  positions. 
Similar  phenomena  are  to  be  observed  in  the  shells  of 
Lamellibranchs.  The  causes  of  the  metallic  colours  of 
birds  has  been  carefully  investigated  by  Gadow,  who 
points  out  that  if  we  look  at  a  feather  in  a  direction 
nearly  parallel  to  its  plane,  having  one  eye  between  it 
and  the  light,  it  appears  black,  as  it  does  also  when 
placed  between  the  eye  and  the  light.  If  we  keep  the 
feather  steady,  and  move  the  eye  from  one  to  the 
other  of  the  just  mentioned  positions,  we  notice  the 
gradual  appearance  of  all  the  metallic  colours  that  the 
feather  is  able  to  display.  It  is  important  to  observe 
that  these  colours  do  not  appear  at  random,  but  that 
the  first  to  be  seen  are  those  that  are  nearest  the  red 
end  of  the  spectrum,  and  the  last  those  that  are  nearest 
the  violet.  No  metallic  feather  ever  exhibits  a  brown 
or  grey  appearance,  or,  in  other  words,  any  colour 
that  is  not  spectral.  These  facts  lead  to  the  belief 
that  the  changeable  metallic  colours  are  due  to  a 
structure  comparable  to  that  of  a  prism  ;  this  structure 
is  formed  by  a  transparent  sheath  of  remarkable 
thinness  (0-00085  mm.  in  Sturnus,  0-0022  mm.  'in 
Galbula),  which  is  either  perfectly  smooth  and 
polished,  or  has  fine  longitudinal  ridges  or  numerous 
minute  dots  on  its  surface.  Below  the  sheath  there 
is  a  brown  or  dark  pigment.  As  a  very  small  part 
of  the  orbit  of  a  curve  may  be  treated  as  a  straight 
line,  the  sheath  may  be  regarded  as  consisting  of  a 
number  of  small  prisms  ;  the  reason  why  colour  is  not 
seen  when  the  eye  is  between  the  object  and  the  light 
is  that  such  a  prism  only  produces  colour  on  the  side 
farthest  from  the  light,  and  therefore  refracts  no  light 
in  the  direction  of  the  observer, 
s— 16 


274 


CHAPTER   IX. 

PROTECTING    AND    SUPPORTING    STRUCTURES. 

WITH  the  exception  of  such  simple  forms  as  Prot- 
amceba,  even  the  lowest  Protozoa  exhibit  some  kind 
of  difference  between  the  outer  parts  of  the  cell  that 
have  to  bear  with  the  jars  and  dangers  of  external 
agencies,  and  the  inner  parts  that  are  protected  from 
them.  In  Amoeba  itself  we  can  recognise  (Fig.  1) 
a  difference  between  the  outer  ectosarc  and  inner  en- 
dosarc.  The  former,  from  our  present  point  of 
view,  may  be  merely  said  to  be  firmer  ;  but  we  have 
to  note  that  the  ectosarc  of  such  Amoebae  as  live  in 
moist  earth  is  much  firmer  than  in  those  which  live  in 
water.  But  the  group  of  which  the  Amo3ba  is  the 
simplest  representative  is  not  without  parts  which 
clearly  form  supports  for  the  protoplasm  of  the  cell  : 
these  are  firmer  structures,  which  may  be  called 
skeletons. 

In  the  present  chapter,  then,  we  shall  chiefly  denl 
with  skeletal  structures,  whether  internal  or  external, 
but  we  shall  have  also  to  speak  of  other  offensive 
and  defensive  organs. 

The  ectosarc  of  the  Amoeba  leads  to  the  definite 
cuticle  of  the  Infusorian;  but  the  Sarcodina  are  nob 
without  external  defensive  structures.  The  simplest 
condition  may  be  found  in  such  a  form  as  Gromia 
(Fig.  Ill),  where  the  ectosarc  forms  an  organic  layer  of 
a  substance  like  chitin,  which,  while  it  envelops  the 
general  body  mass,  may  be  itself  flowed  over  by  the 
contained  protoplasm.  In  such  a  test  as  this  there  is 
a  single  large  orifice  at  one  end.  This  chitinous  test 
varies  considerably  in  thickness  and  consistency  in 


Chap,  ix.]        SKELETONS  OF  RHIZOPODA. 


275 


various  Rhizopods,  and  may  take  on  the  most  different 
forms,  and  even  become  pigmented.  In  marine 
Rliizopods  the  test  becomes  much  firmer,  owing  to  the 
deposition  in  its  substance  of  calcareous  salts  ;  and  as. 


Fig.  111.— Gromia  tenicola,  showing  the  Protoplasm  extending  round 
the  chitinous  test.     (After  Leidy.) 

the  test  becomes  traversed  by  pore  canals,  through 
which  there  pass  processes  of  the  protoplasmic  body 
that  has  formed  it,  we  get  a  structure  which  more 
easily  falls  in  with  our  idea  of  a  skeleton.  This 
skeleton  may  be  rounded  and  simple,  or  else  it  may 
give  off  fine  projecting  processes,  or  it  may,  as  in 
Orbitolites  and  other  Foramiiiifera,  become 


276    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

very  elaborately  coiled,  and  attain,  as  in  the  fossil 
JVummulites,  to  a  considerable  size  (more  than  four 
inches  in  diameter  ;  or,  as  in  some  recent  species,  to 
a  diameter  of  two  inches).  Other  Rhizopods  build  up 
their  skeleton,  as  do  many  sponges,  from  the  silica 
dissolved  in  sea-water,  and  others,  like  a  number  of 
sedentary  worms,  take  up  sand  and  other  foreign 
products,  and  weld  them  into  a  consistent  skeleton. 

In  the  Heliozoa  there  may  be  a  more  or  less 
gelatinous  investment,  which,  as  in  the  Rhizopoda, 
may  appear  at  times  only,  or  be  permanent ;  or  there 
may  be  a  definite  skeleton,  which  is  in  no  case  calca- 
reous. It  is  most  often  formed  of  silex,  and  its  parts 
are  often  disconnected.  In  rare  cases  a  shell  is  formed 
of  sand  only,  or  of  sand  and  the  tests  of  diatoms. 

The  Racliolaria  are  remarkable  for  the  pos- 
session of  a  so-called  "  central  capsule,"  which  is 
membranous  in  structure,  and  is,  like  the  test  of 
Gromia,  perforated  at  one  point  only,  where  there  is 
a  comparatively  large  space,  or  the  membrane  is  per- 
forated by  several  spaces,  or  a  number  of  pore  canals 
(as  in  the  test  of  the  perforate  Foraminifera).  In 
addition  to  this  membranous  central  capsule,  most, 
though  not  all,  Radiolaria  have  also  a  skeleton  which 
may  or  may  not  penetrate  the  central  capsule.  This 
skeleton  is  made  up  of  spicules,  which  either  consist 
of  an  organic  substance,  acanthin,  as  in  the  Acantho- 
metridae,  or  of  a  siliceous  compound.  These  spicules 
are  primarily  arranged  in  a  radiating  fashion,  and  are 
often  connected  by  secondary  spicules  with  one 
another,  the  result  being  forms  of  the  utmost  delicacy, 
and  of  great  beauty  (Fig.  112). 

The  great  variety  of  skeletal  structure  which  is 
seen  in  the  Sarcodina  does  not  obtain  in  the  Infusoria, 
many  of  which  are  extremely  active  in  movement.  In 
various  divisions,  however,  we  find  that  the  cuticle  be- 
comes particularly  hard,  and  the  so-called  lorica  (Fig, 


Chap.  IX.] 


XlPHACANTHA. 


277 


113)  thus  formed  may  be  variously  ornamented;  the 
stalk,  well  known  in  Vorticella,  is  not  contractile  in 
Epistylis  and  others,  The  lorica  may  be  produced 


Fig.  112,  -XivhacantJia  murrayana.    (After  Wyville  Thomson.) 

into  tooth-like  or  tail-like  processes  ;  a  shield-shaped 
test,  or  a  bivalved  carapace  may  be  developed,  or  the 
body  may  become  surrounded  by  a  gelatinous  capsule. 
Karely,  as  in  the  Dictyocyrtidse,  the  investing  test 
becomes  impregnated  with  siliceous  bodies. 


278    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Even  naked  Protozoa  may  become  covered  with  a 
firm  cyst  formed  by  the  ectosarc,  at  such  times,  as  from 
choice  or  necessity  they  pass  into  a  quiescent  con- 
dition. This  power  of  eneystatioii  is  found  also  in 
the  lowest  members  of  the  vegetable  kingdom,  and  is 
a  means  of  protection  for  the  protoplasm  at  the  time 
that  it  is  undergoing  the  important  changes  that  pre- 
cede the  rejuvenescence  of  the  indi- 
vidual, or  the  production  of  progeny. 

It  is  impossible  to  pass  from  the 
Protozoa  without  reminding  the  stu- 
dent of  how  large  and  important  a  part 
they  have  played  and  are  playing  in 
the  formation  of  the  earth's  crust.  The 
aphorism  of  Linnaeus,  "  Petrefacta 
montium  calcariorum  non  filii  sed 
parentes  sunt,  cum  omnis  calx  oriatur 
Fig.  us.— Tintin-  ab  animalibus,"  is  supported  by  our 

nus     lagenula,  .       ,       -,-,-,  -, 

showing  the  recently  acquired  knowledge  that 
and  the  Or ovm  Diatoms  and  Globigerinae  live  on  th, 
of  Cilia.  surface  of  the  sea,  and  that  their  case.s 

and  tests  sink  to  the  bottom  when 
their  inhabitants  and  makers  die.  Some  rocks, 
such  as  chalk-cliffs,  are  full  of  the  tests  of  Grlobigerinse, 
and  the  "  Nummulitic  Limestone "  of  Nummulites. 
Casts  of  Foraminifers  have  been  found  in  greensand  ; 
a  silicate  of  iron  and  alumina  has  been  found  filling 
casts  of  recent  Foramlnifera,  so  that  as  a  matter  of  fact 
we  at  this  present  period  find  "  greensand  replacing 
and  representing  the  primitively  calcareo-siliceous 
ooze ; "  and,  lastly,  the  researches  of  the  Challenger 
show  that  at  a  depth  greater  than  2,500  fathoms  a 
substance  known  as  "  red-clay  "  takes  the  place  of  the 
Globigerina  ooze. 

In  all  but  the  lowest  Sponges  (Myxospongiae) 
skeletal  structures  have  been  observed,  and  these,  as  in 
the  Protozoa,  are  of  an  organic  nature  simply  (fibrous 


Chap.  IX.j 


SPICULES  OF  SPONGES. 


279 


sponges),  or  the  organic  substance  becomes  impregnated 
with  calcareous  salts  (calcareous  sponges),  or  with 
siliceous  (siliceous  sponges).  Considerable  variations 
are,  moreover,  to  be  seen  in  the  extent  to  which  this 
impregnation  takes  place,  so  that  while  the  fresh- water 
sponge  (Spongilla)  has  but  few  and  simple  siliceous 
spicules,  the  Lithistidse  are  quite  hard  and  strong. 
In  most  cases  the 
inorganic  skeleton  is 
spicular,  and  not 
continuous ;  but  in 
some,  as  "  Venus's 
Flower  Basket :' 
(Euplejtella),  a  deli- 
cate framework  of 
siliceous  particles  is 
left  after  all  the  or- 
ganic material  has 
been  removed  (Fig. 
114). 

The  spicules  vary 

•  i        i_i     •     £  Fig.  114.— Section  through  the  Wall  of 

considerably  in  form,  Euplecteila  ( x  75). 

being      uniaxial      Or     p,   Pores;   -f,    flagellated    chambers.     (After 

needle-shaped,  tri- 
axial  (this  is  the  cha- 
racteristic form  in  the  Calcispongiae),  or  quadriaxial ; 
connected  with  these  are  bi,  tri,  quadri,  and  sex- 
radiate  spicules,  which  may  by  the  loss  of  some,  and 
the  greater  development  of  other  rays,  take  on  the 
most  different  shapes.  Some  spicules  are  multi-radiate, 
and  others  curved.  Some  project  beyond  the  body  of 
the  sponge,  as  in  the  glass-rope  sponge  (Hyalonema ; 
Fig.  115),  where  anchoring  spicules  as  much  as 
eighteen  inches  long  have  been  observed.  In  addi- 
tion to  these  proper  skeletal  spicules,  others  which 
are  smaller  take  an  important  part  in  giving  firmness 
to  the  sponge  body,  and  even,  as  in  the  case  of  tho 


280    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


"  gemmules  "  of  the  fresh- water  -sponge  forming  the 
"  amphidiscs,"  which  strengthen  its  protective  coat 
during  the  period  of  quiescence.  Sponges  free  from 
calcareous  or  siliceous  spicules,  and  with 
only  a  fibrous  skeleton,  have,  in  the 
present  period,  some  commercial  value, 
in  consequence  of  their  well-known  use 
to  man. 

From  the  share  that  they  have  had 
in  forming  parts  of  the  earth's  crust, 
there  is  no  division  of  the  animal  king- 
dom in  which,  from  such  a  point  of  view, 
skeletal  structures  are  of  more  impor- 
tance than  in  the  Ccelenterata,  and  both 
Hydrozoa  and  Anthozoa  contain  groups, 
members  of  which  form  the  hard  struc- 
tures which  we  call  coral ;  this  con- 
sists essentially  of  deposits  of  carbonate 
of  lime  in  the  organic  substance  of  the 
body. 

The  division  of  the  Anthozoa  con- 
tain the  larger  number  of  coral-forming 
animals,  and  may  therefore  be  first  dealt 
with.  In  the  simplest  forms,  such  as  the 
common  sea  -  anemone,  there  are  no 
spicules  at  all.  but  the  body  wall  is  ren- 
dered more  or  less  consistent  by  the 
development  of  fibrils  of  connective 
tissue  in  the  mesoderm  ;  this  may  be 
called  the  supporting  lamella,  and,  as  we 
it  is  thinner  in  the  tentacles  than  in 
the  rest  of  the  body,  where  it  may  become  thrown 
into  folds ;  from  the  body  wall  bands  or  septa,  in 
the  midst  of  which  is  a  more  or  less  thin  support- 
ing lamella,  project  inwards,  and  some  of  them  reach 
the  wall  of  the  gastric  cavity  which  lies  in  the  central 
axis  of  the  polyp.  (See  Fig.  54.)  In  rare  cases  the 


Fig.  115.—  Hya* 
lonema  sie- 
boldi.  (After 
Schultze.) 


may  suppose, 


Chap.  IX.] 


SPICULES  OF  ANTHOZOA. 


28! 


non-spiculate  Anthozoa  take  up  foreign  bodies  into,  and 
thereby  strengthen,  their  ectoderm. 

Thenext  stage  isseen  in  Alcyomum,  where  definitely 
formed  but  scattered  spicules  are  found  in  the  layers 
•of  connective  tissue.  Where  the  skeleton  becomes 
continuous  it  may  be  horny,  and  where,  as  in  Gorgonia , 
a  number  of  polyps  are  connected  together,  the  skele- 
ton of  the  common  trunk  forms  a  horny  axis ;  in  the 
mesodermal  tissues  of  the  polyp  spicules  with  an 


Fig.  116.  —A,  Triaxial  spicule  of  Calcisponge  (Ascetta  blanca)  ;  B,  Simple 
Acerate  Spicule  of  Reniera ;  c,  Six-rayed  Spicule  of  the  Hexactinel- 
lidse. 

organic  basis  are  developed,  which,  on  the  death  of  the 
animal,  merely  form  a  crust  on  the  axis.  In  Isis  the 
axis  is  calcined  at  certain  points  only  ;  so  that  it  is 
alternately  horny  and  calcareous.  In  the  red  coral  the 
whole  of  the  axis  is  calcined  (Fig.  117).  In  other 
cases,  as  in  the  organ-pipe  coral,  the  hard  deposits  are 
laid  down  in  the  wall  only  of  the  polyp  (Fig.  118  A), 
and  these  tubes  become  connected  with  their  neigh- 
bour by  lateral  outgrowths,  and  so  form  a  continuous 
hard  mass.  In  others,  as  in  the  only  "  coral "  found 
on  our  own  shores  (Caryophyllia)  the  deposits  in 
spicules  is  not  confined  to  the  wall,  but  extends 
also  into  the  septa  (Fig.  118  B)  ;  in  others,  as  in 


282    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Fig.  117.— Section  of  Axis  of  the  Ked  Coral,  magnified. 


Chap.  IX.] 


PARTS  OF  CORALS. 


283 


the  common  Fungia,  the  spicules  are  found  in  the 
septa  only,  while  the  body  wall  remains  soft. 
Finally,  the  axis  common  to  a  colony  of  polyps  may 
become  calcined  as  well  as  the  body  wall  and  the 
septa  (Fig.  118  c),  and  we  then  get  large  masses 
of  hard  stony-like  structures  which  persist  long  after 
the  polyps  that  formed  them  are  dead  and  decayed 
(brain-coral). 

In  describing  the  skeletons  of  corals  use  is  made 
of  the  following  terms  :  the  wall  of 
the  cup-like  calcification  is  called  the 
theca,  and  consists  sometimes  of 
an  exo-  and  endo-  theca;  where  the 
theca  is  thin  it  is  aided  by  an  in- 
vesting epitliecsi ;  the  space  be- 
tween the  calcined  septa  are  the 
loculi ;  the  septa  may  unite  in  the 
centre  to  form  a  pseudo-colum- 
ella,  or  may  be  inserted  into  an 
axial  hard  part  of  distinct  origin, 
which  is  the  true  columella;  the 
ridges  or  outgrowths  on  this  are  the 
pali,  and  the  synapticulse  are  the 
plates  that  project  transversely  and 
connect  one  septum  with  another, 
sometimes  divided  into  chambers  which  rise  one  above 
the  other,  like  storeys,  and  the  floor  of  each  of  these 
is  a  tabula.  The  ridges  on  the  exotheca  are  known 
as  costse. 

The  hard  connecting  sclerenchyma  may  be  compact, 
as  in  the  stony  corals,  or  traversed  by  canals,  as  in  the 
red  coral  (Fig.  117). 

Among  the  Hydrozoa  continuous  coralla  are 
found  only  in  the  Hydrocorallinre,  where  they  are 
formed  by  the  ectoderm  which  covers  the  canals  that 
traverse  and  branch  in  the  "  ccenosarc,"  that  is 
common  to  the  compound  stock  of  polyps ;  the 


Fig.  118  A.  —  Coral. 
Two  tubes  of 
Tubipora  musica, 
•with  their  con- 
tained polyps. 

The   loculi   are 


284    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

gastric  cavities  of  the  separate  polyps  communicate 
with  these  canals.  The  coralla,  though  porous,  are 
very  hard  and  stony,  and  the  canals  are  separated  into 
storeys  by  tabula,  and  the  upper  chambers  are  alone 
living.  The  Millepores  are  common  on  coral  reefs,  the 


Fig.  118  B.— Coral.     Cari,ophyUia  cyathus. 

Styl  asters  live  in  water  from  ten  to  seven  hundred 
fathoms  in  depth. 

In  all  the  other  Hydrozoa  the  supporting  tissue  is 
simply  a  supporting  lamella  mesodermal  in  position, 
as  in  the  common  Hydra,  or  there  is  an  outer  chitinous 
perisarc,  as  in  many  Hydroid  polyps,  which  persists 
after  the  death  of  the  animal  as  the  so-called  coralline  ; 
or,  as  in  the  Medusae,  the  tissue  lying  between  the 
ectoderm  and  endoderm  becomes  gelatinous  (Fig.  119), 
or  cartilaginous. 

Spicular  skeletons  are  not  found  among  Vermes, 
where,  when  a  protecting  tube  is  developed,  it  is  often 


Chap,  ix.]  DENDROPHYLLIA.  285 

largely  composed  of  foreign  material ;  nor  is  there  any- 
complete  internal  skeleton.     The  cuticle  may  be  soft 


.Fig.  118  c. — Coral.     Dendrophyllia  ramosi. 

and  ciliated,  as  in  the  Tiirbellaria,  or  become  very 
firm  and  appear  to  be  formed  of  a  chitiiious  substance, 
as  in  the  Nematoidea,  or  still  more  in  the  Rota- 
toria,  where  it  maybe  join  ted  and  have  muscles  inserted 


286    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


into  it.  In  the  Animlata  it  often  becomes  of  con- 
siderable thickness  and  is  then  traversed  by  pore-canals. 
In  the  sedentary  marine  Annelids  a  tube  is  developed 
as  a  means  of  protection ;  the  inner  portion,  which  is 

partly  membran- 
ous and  partly 
fibrillated,  is 
formed  by  special 
glands  in  the 
body  wall ;  out- 
side this  the  tube 
is  often  rendered 
moreresistent  by 
the  deposition  of 
calcareous 
matter  (as  in 
Serpula),  or  of 
aggregations  of 
sand,  mud,  and 
other  foreign  ma- 
terial (as  in  Sa- 
bella,  or  Amphi- 
trite),  which  are 
taken  up  by  the 
tentacles  of  the 
worm,  and  laid 
down  on  the  tube 
by  the  animal  it- 
self. Within  this 
tube  the  inhabitant  may  be  retracted,  and  some  (as 
Sabella)  form  an  operciilum  by  means  of  which  the 
entrance  to  it  may  be  closed. 

In  the  Sabellidse  special  cartilaginous  supports  are 
developed  within  the  gill  tentacles ;  this  is  not  found 
in  the  Serpulidse. 

The  cells  of  the  integument  often  give  rise  to  hard 
projecting  structures,  which  may  have  the  form  of 


Fig.  119.— Gelatinous  Tissue  from  the  Disc  of 
Aurelia  aurita;  a,  fibres:  b,  cells.  (After 
M.  Sclxultze.) 


Chap.  IX.] 


HOOKS  OF  CESTODA. 


287 


hooks,  or  of  bristles.     The  former  are  well  developed 
in  some  of  the  Cestoda,  as   in  Taenia  solium    (the 


Pig.  120. — tierpula  vermicular™,  showing  tlie  coiled  Tube,  and  the 
Animal  protruded. 

tapeworm),  where  the  head  is  surrounded  by  a 
circlet  of  recurved  chitinous  hooks,  by  means  of  which 
the  animal  fixes  itself  to  the  mucous  membrane  of  the 
intestine  of  its  host ;  the  presence  of  these  is  the  cause 
of  the  difficulty  found  by  the  practitioner  in  attempting 
to  expel  the  parasites  from  the  human  intestine. 


288   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY, 

Tapeworms  with  hooked  heads  are  found  in  carnivorous 
mammals  and  in  birds,  where  the  cavity  of  the  in- 
testine is  comparatively  limited,  but  they  have  not 
yet  been  seen  in  such  Cestoda  as  live  in  herbivorous 
mammals,  where  the  intestinal  tract  is  much  more 
spacious.  The  group  term  Acanthocephali,  and  the 
generic  name  Echinorhynchus,  refer  to  the  presence  of  a 
number  of  hooks  on  the  "  proboscis  "  of  other  parasites. 

In  the  higher  Nemertinea  stylets  are  developed 
at  the  base  of  the  proboscis,  and  it  is  particularly 
interesting  to  observe  that,  where  not  present,  their 
place  is  taken  by  stinging  cells  ;  a  similar  correlation 
is  found  among  the  Turfoellaria,  where  the  absence  of 
nematocysts  is  often  atoned  for  by  the  presence  of 
small,  rod-like  structures,  the  so-called  rtiafodites. 
In  the  Chsetopoda  some  of  the  gland-cells  of  the 
integument  secrete  hard  chitinous  bristles  or  setae  of 
various  lengths,  which  are  protective  and  locomotor 
organs  ;  in  the  Oligochaeta  (e.g.  Lumbricus,  the 
earthworm)  these  setse  are  few  in  number,  and  never 
exceed,  so  far  as  is  known,  eight  in  all ;  in  the  marine 
Polychseta  they  may  be  more  numerous  and  much 
larger  than  in  the  earthworm  ;  they  may  be  variously 
denticulated  or  hooked  at  their  free  ends,  and  may, 
in  the  tube  dwellers,  aid  the  animal  in  raising  itself 
up  its  tule. 

The  Polyzoa  are  provided  with  an  organ  of  pro- 
tection, which  is  in  all  cases  external  or  of  tegumentary 
origin  ;  it  may  be  soft  and  gelatinous,  or  harder  and 
chitinous,  or  calcareous.  It  has  been,  somewhat 
unfortunately,  called  a  cell ;  it  invests  only  the  hinder 
part  of  the  body,  but  it  may  serve,  in  times  of  danger, 
as  a  refuge  for  the  more  anterior  portion,  which  can 
be  withdrawn  into  it. 

All  Echinoderms,  with  the  exception  of  the 
Holothuroidea,  have  a  well-developed  skeleton,  and 
such  is  found  also  in  some  Holothurians.  It  is  formed 


Chap.  !£.]  TEST   OF   ECHINOIDEA.  289 

of  an  organic  basis,  which  becomes  impregnated  with 
calcareous  salts,  and,  in  thin  sections,  has  a  ver} 
characteristic  reticular  appearance. 

It  is  particularly  well  developed  intheEchinoidea, 
with  the  consideration  of  which  it  will  be  convenient 
to  commence.  In  recent  forms  the  test  ^corona) 
is  made  up  of  ten  pairs  of  rows  of  plates,  five  of  which 
are  radial  and  five  interradial  in  position;  the 
former  are  perforated  at  the  outer  edge  to  allow  of  the 
passage  of  the  ambulacral  tubes  or  suckers  ;  in  the 
fossil  Palcechinoidea  the  interambulacral  plates  were 
not  paired,  but  as  many  as  five  or  six  took  the  place 
of  the  two  which  are  now  constantly  developed  in  all 
known  living  species.  These  plates  of  the  corona, 
which  are  covered  by  an  epithelial  lining  and  by  the 
extracoronal  portion  of  the  peripheral  nervous 
system,  are  ordinarily  firmly  attached  to  one  another, 
so  that  no  part  of  the  corona  is  movable ;  in  some, 
however,  such  as  Asthenosoma,  the  plates  are  mov- 
able on  one  another,  and  the  whole  test  is  flexible. 

The  rows  of  pores  may  remain  straight,  as  in  Cidaris, 
or  three  or  more  primary  may  unite  to  form  larger 
secondary  plates,  and  the  pores  then  become  arranged 
in  arcs  ;  three  pairs  of  pores  go  to  form  an  arc  in  an 
Echinus,  and  as  many  as  twelve  or  thirteen  in  a 
Heterocentrotus.  The  plates  carry  tubercles  of  vary- 
ing sizes,  and  on  these  tubercles  (Fig.  121;  B)  are 
placed  movable  spines,  which  may  be  quite  short,  as 
in  Echinus,  longer  than  the  long  axis  of  the  body,  as 
in  the  piper  (Dorocidaris),  or  very  strong  and  massive, 
as  in  Heterocentrotus.  Sometimes,  as  in  Diadem  a, 
these  spines  are  not  only  protective  organs  in  virtue  of 
their  own  strength  and  number,  but  are  also  capable 
of  inflicting  painful  burning  wounds  in  a  manner  which 
has  not  yet  been  satisfactorily  explained.  Sometimes, 
as  in  Spatangus  or  Echinocardium,  the  spines  become 
very  fine  and  silky.  In  most,  though  not  in  all  cases 
T— 16 


290  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  larger  primary    are  surrounded    by   smaller  and 
more    delicate    secondary    spines.       On     the     lower 


(actinal)  surface  of  the  corona  there  is  an  orifice  which 
is  the  mouth,  on  the  upper  there  is  a  special  arrange- 
ment of  plates  which  form  the  so-called  apical  area. 


Chap,  ix.]         APICAL  AREA  OF  ECHINUS.  291 

This  is  best  studied  in  a  regular  form  such  as  the 
ordinary  sea-urchin  (Echinus),  where  it  is  found  to 
consist  of  two  sets  of  five  plates  (Fig.  121  ;  A),  one  of 
which  is  radial  and  one  interradial  in  position.  The 
former  are  spoken  of  as  the  ocular  plates,  and  are 
perforated  by  an  ordinarily  single  orifice,  through 
which  a  tentacle  protrudes.  The  interradial  plates, 
which  are  similarly  perforated,  and  which  are  generally 
larger  than  the  radial,  are  called  the  genital  plates, 
from  the  fact  that  they  have  become  secondarily 
modified  to  serve  as  the  means  of  exit  of  the  generative 
products  ;  for  the  comprehension  of  the  morphology  of 
this  apical  part  of  the  test  it  is,  however,  best  to 
strictly  limit  our  nomenclature  to  the  terms  radial  and 
interradial.  One  of  these  interradial  plates,  that 
which  lies  to  the  right  of  the  anterior  ambulacrum  or 
radial  plate,  as  seen  in  Fig.  121,  is  specially  modified 
to  serve  as  the  entrance  to  the  madreporic  or  stone- 
canal.  It  is  distinguished  by  the  name  of  madrepo- 
rite,  and  is  characterised  by  its  larger  size,  and  its 
perforation  by  a  number  of  minute  orifices. 

Within  the  circlet  of  these  radial  and  interradial 
plates  of  the  apical  area,  there  is  a  space  which  is 
ordinarily  covered  by  a  number  of  small  irregularly- 
shaped  calcareous  plates,  in  or  near  the  middle  of 
which  there  is  an  orifice,  the  anal  opening  of  the 
digestive  tract ;  sometimes,  as  in  Echinocidaris,  the 
number  of  anal  plates  is  much  smaller,  and  in  the 
genus  just  mentioned  there  are  very  often  not  more 
than  four  ;  in  Diadema  the  anal  area  is  very  nearly 
completely  membranous,  and  the  anal  orifice  is  placed 
at  the  end  of  a  projecting  tube.  Among  the  irregular 
echmids  the  anus  leaves  its  apical  position,  and  opens 
either  some  way  posteriorly  on  the  upper  surface,  as 
in  Rhyncopygus,  or  at  the  margin  of  the  test,  as  in 
Echinolampas,  or  on  the  lower  surface  and  quite  close 
to  the  mouth  as  in  Echiiioneus. 


292  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  region  of  the  anus  in  the  regular  echinoids  is 
primitively  occupied  by  a  single  large  plate,  the 
dorso-central,  and,  in  the  study  of  the  morphology  of 
the  apical  area  of  Echinoderms,  it  is  necessary  to  always 
bear  in  mind  (L)  the  dorso-central,  (2)  the  radial,  and 
(3)  the  basal  plates,  which  are  interradial  in  position. 

Within  the  test  of  the  Echinus  are  five  calcareous 
arches  (auriculae)  which  afford  attachment  to  the 
"Lantern  of  Aristotle";  these  auricles  are,  when 
present,  radial  in  position  in  all  Echinoids,  except  the 
Cidaricla,  where  they  are  interradial. 

In  the  adult  Crinoid  we  distinguish  a  cup,  or 
calyx,  which  may,  as  in  Pentacrinus,  be  permanently 
fixed  by  a  stalk,  or,  as  in  Antedon,  be  stalked  in  the 
larval  stages  only.  In  both  cases  the  calyx  gives  off 
a  number  of  arms,  which  consist  of  numerous  small 
calcareous  joints,  and  have  jointed  appendages,  the 
pinnules,  attached  to  them.  However  numerous 
these  arms  may  be,  and  there  may  be  almost  ono 
hundred,  we  find  that,  as  we  trace  them  back  to  the 
calyx,  they  form  branches  of  one  or  other  of  its  five 
rays.  These  rays  ordinarily  consist  (the  common 
Antedon  of  our  own  seas  is  an  excellent  example)  of 
three  radial  joints  ;  all  these  joints  unite  to  a  common 
central  piece,  which  is  known  as  the  centro-dorsal, 
In  the  stalked  forms  this  centro-dorsal  is  placed  at  the 
top  of  the  stalk,  which  consists  of  a  large  number  of 
small  ossicles,  and  is  at  various  points  along  its  length 
provided  with  jointed  five-rayed  outgrowths  (cirri), 
which  have  a  claw  at  their  free  end.  In  Antedon  and 
others,  where  the  stalk  is  lost  in  adult  life,  the  cirri, 
which  vary  a  good  deal  in  number,  and  are  sometimes, 
after  a  certain  age,  completely  lost,  are  directly  attached 
to  the  centro-dorsal  itself. 

We  apply  the  term  centro-dorsal  to  the  central 
plate  of  the  crinoidal  calyx  to  distinguish  it  from  the 
dorso-central  of  the  typical  unaltered  apical  area  of 


Chap.  IX.  ] 


CALYX  OF  CRINOIDS. 


293 


Echinoderms ;  the  plate  is,  in  the  Crinoids,  modified 
by  the  large  share  in  its  formation  that  is  taken  by  the 
topmost  plate  of  the  stalk. 
In  the  development  of  An- 
tedon,  the  interradially- 
placed  tmsals  become  ob- 
scured, but  in  other  forms 
among  the  Crinoids  they  are 
evident  throughout  life. 
Careful  investigation  into  the 
structure  of  the  skeleton  of 
Asteroids  and  Ophiuroids 


Fig.  122.— Pentacrinus  Wyville-Thomsoni.     (Natural  size.) 

reveals  the  presence  of  the  plates  to  which  our  atten- 
tion has  just  been  directed. 

The  following  are  the  more  important  points  in 
the  structure  of  the  skeleton  of  the  Starfish;  the 
arms,  or  rays,  are  made  up  of  a  number  of  ossicles 


294  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


set  in  regular  paired  rows,  at  a  more  or  less  acute 
angle  to  one  another  ;  these  ossicles  are  not  perforated 
like  the  ambulacral  plates  of  an  echinid,  but  the 


Ir 


T  V-v  \7        I 

$M*& 

Vr 

^^x^=^v         ^\  \ 


Fig.  123  A. — Cross  Section  of  an  Arm  of  a  Starfish  (Asterias  rubcns). 
On  the  left  side  the  section  is  supposed  to  pass  between  two  of  the  ambulacral 
ossicles,  but  on  the  right  side  through  one  of  them  (ao) ;  ap,  ambulacral 
groove;  n,  radial  nerve;  6,  radial  blood-vessel;  u\  radial  water-vessel;  a, 
ampullae;  t,  tentacles  or  suckers  ;  ap,  adambnlacral  plates;  s.p,  spines  ;  pax, 
paxillse,  arising  from  limestone  plates ;  ov,  ovary ;  ftp,  genital  pore ;  gv, 
genital  blood-vessel ;  6r,  respiratory  processes ;  pc,  cseca  of  the  intestine. 
(After  P.  H.  Carpenter.) 

tube-feet  pass  out  between  them  ;  attached  to  each 
ambulacral  ossicle  is  a  smallei  ad -ambulacral  ossicle 
(Fig.  123  A;  ap\  which  completes  the  side  of  the 
groove  and  carries  spines ;  the  rest  of  the  wall 
of  the  arm  is  strengthened  by  irregular  plates, 
which  may  be  so  formed  as  to  leave  considerable 
interspaces,  as  in  the  starfish,  or  they  may  be  larger 
and  more  closely  packed,  and  have  only  minute  pores 
between  them,  as  in  Linckia ;  sometimes,  as  in 
Oreaster,  the  plates  that  form  the  margin  of  the 
arm  form  two  regular  rows  of  much  stronger  supero- 
aiid  infero-margiiial  plates.  All,  some,  or  none 


chap,  ix.]         SKELETON  OF  ECHINODERMS. 


295 


d.o 


of  these  ossicles  may  bear  spines  of  varying  size  and 
strength  ;  where  they  are  best  developed  we  rarely 
find  that  any  of  the  arms  have  suffered  injury,  and 
they  are,  no  doubt,  of  very  considerable  importance 
as  protective  organs.  The  disc  is  formed  chiefly  of 
irregularly  arranged  intermediate  plates,  but  the 
radials  and  basals  are,  in  some,  cases,  to  be  clearly 
detected  in  young  specimens  ;  near  or  at  the  centre  of 
the  disc  there  is  an  anal  perforation  which  is  rarely 
wanting  (  Astropecten).  On  the 
lower  surface  the  large  central 
mouth  is  to  a  slight  extent 
aided  in  its  work  by  the  modi- 
fication of  the  most  central 
ambulacral  ossicles,  which  pro- 
ject inwards  at  the  angles  of 
the  mouth. 

In  the  Ophiuroidea  a 
cross  section  of  the  arms  (Fig. 
123  B)  shows  that  the  ambu- 
lacral ossicles  (ao)  are  covered 
in  on  all  sides,  so  that  no 
groove  is  apparent;  pores  in 
the  lower  plate  allow  of  the 
passage  of  the  tube  feet  ;  the  side  plates  («)  ordinarily 
bear  spines  (t),  which  are  never  of  great  length  or  much 
size,  and  can  be  of  little  use  as  organs  of  defence  ; 
above,  a  single  plate  (u)  roofs  in  the  ambulacral  ossicles, 
but  this  is  rudimentary  in  Neoplax,  and  absent  in 
Ophioscolex.  The  plates  in  the  disc  are  propor- 
tionately larger  than  in  starfishes,  are  ordinarily  set 
in  a  close  mosaic,  and  not  unfrequently  exhibit  the 
essential  parts  of  the  typical  calyx,  the  dorsocentral 
even  being  often  apparent,  owing  to  the  fact  that 
there  has  been  no  resorption.  of  calcareous  tissue  to 
make  room  for  an  anal  orifice.  The  plates  around  the 
mouth  are  so  arranged  as  to  give  rise  to  five  radially 


(  After  p.  H.  Carpenter.) 


296  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

arranged  slits  ;  the  edges  of  these  slits  often  bear 
small  spines,  while  the  oral  faces  of  the  ossicles  carry 
similar  spines,  the  so-called  oral  papillae ;  this 
armature  of  fine  spines  serves  no  doubt  as  a  filtering 
apparatus  to  the  digestive  cavity  of  these  aproctous 
Echinoderms.  We  may  correlate  the  injuries  which 
the  arms  are  often  seen  to  undergo  with  the  absence 
of  defensive  spines ;  the  Ophiuroid  leaves  an  arm  with 
the  foe  from  which  it  is  unable  to  defend  itself ;  and 
we  may  compare  with  this  the  arrangements  of  the 
tail  vertebrae  of  the  harmless  lizard.  (See  page  322.) 
Arms  thus  broken  are  in  time  renewed.  While  in 
the  Ophiurida  the  arms  are  nearly  always  un- 
divided, however  long  they  may  be,  the  Astrophytida 
exhibit  various  stages  of  division  ending  in  the  great 
complexity  of  the  free  termination  of  the  arms  which 
obtain  in  the  basket-fish,  or  gorgon's  head  (Astro- 
phyton,  Gorgonocephalus) ;  in  the  Astrophytida  the 
spines  are  reduced  to  a  minimum,  and  the  integument 
is  thick  and  leathery. 

Though  many  Holothiirians  have  a  very  thick 
skin,  and  a  deposit  only  of  spicules  in  their  integu- 
ment, we  cannot  suppose  that  this  is  a  retention  of 
the  primitive  condition,  spoken  to  by  the  fact  that  in 
all  Echinoderms  the  skeleton  commences  in  the  form 
of  spicules,  which  gradually  unite  mere  or  less  with 
one  another,  so  much  as  one  that  lias  been  secondarily 
acquired.  In  some  cases  (Psolus)  the  calcareous 
plates  are  quite  large,  firm,  and  connected,  and,  on 
the  other  hand,  the  spicules  sometimes  disappear 
completely  from  old  and  large,  even  where  they  are 
present  in  younger  and  smaller,  examples  of  some 
species  of  Cucumaria.  In  Synapta  the  spicules  take 
on  the  form  of  anchors ;  in  Chirodota,  of  toothed 
wheels.  They  are  often  turn-form  in  shape,  and  the 
surface  of  the  body  is  sometimes  quite  rough  to  tfie 
touch,  owing  to  the  large  numbers  which  are  present 


Chap,  ix]  PEDICELLARI&.  297 

in  and  project  from  the  integument.  In  these 
Echinoderms,  where  the  defensive  powers  of  the 
skeleton  are  slight  or  lost  altogether,  we  again  observe 
that  the  creature  is  prone  to  acts  of  self -mutilation, 
not  unfrequeiitly  ejecting,  when  attacked,  the  whole 
of  its  viscera ;  these  are  in  time  repaired,  if  the 
animal  is  left  to  recover. 

In  the  Echinoidea  and  Asteroidea  a  number 
of  the  spines  are  not  unfrequeiitly  converted  into 
stalked  or  sessile  snapping-like  organs,  the  pedi- 
rcllai'isr,  as  they  were  called  by  those  who  be- 
lieved them  to  be  independent  and  parasitic  animals ; 
the  sessile  pediceilarise  are  bivalve ;  the  stalked 
have  three  or  four  valves  ;  they  are  supported 
by  the  calcareous  reticular  tissue  which  is  so 
characteristic  of  the  hard  parts  of  Echinoderms, 
and  are  moved  by  muscles.  Their  chief  function 
appears  to  be  that  of  holding  on  to  objects  that  come 
into  contact  with  them,  or  to  such  supports  for  the 
progression  of  the  animal  as  waving  fronds  of  sea- 
weed, until  the  suckers  are  able  to  be  brought  into 
relation  with  the  object;  it  has  been  observed  that 
their  prehensile  power  only  lasts  for  about  two 
minutes.  In  some  cases  it  is  probable  that  some  of 
the  pedicellarisc  are  used  for  the  purpose  of  cleansing 
the  neighbouring  spines  of  foreign  or  faecal  material ; 
but,  if  we  are  to  judge  from  the  great  differences 
which  obtain  in  their  number,  and  their  complete, 
or  almost  complete  absence  from  some  species,  the 
close  allies  of  which  have  a  large  number,  we  are 
led  to  believe  that  their  function  is  not  important, 
and  that  they  have  an  inverse  ratio  of  development 
to  the  size  and  number  of  the  spines  proper  (Fig. 
121  ;  c,  D). 

The  Arthropoda  are  as  definitely  characterised 
by  the  development  of  a  chitinous,  as  are  the  Echino- 
dermata  by  that  of  a  calcareous  skeleton;  this  is 


298  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

likewise,  in  large  part,  external,  so  that  while  the 
endoskeleton  of  Vertebrates  is  characterised  by  having 
the  muscles  external  to  it,  the  exoskeletal  parts  of 
an  Arthropod  are  moved  by  muscles  that  lie  inter- 
nally to  them.  The  chitinous  skeleton  may  be  thin 
and  soft,  as  in  the  simpler  E lit 01110 straca,  or 
the  parts  of  the  different  somites  may  fuse,  to 
form,  as  in  the  Crayfish,  a  firmer  cephalothoracic 
carapace  ;  or,  as  in  the  Ostracoda  and  other  Ento- 
mostraca,  give  rise  to  two  more  or  less  dense  lateral 
valves ;  in  other  cases  certain  parts  may  become 
very  strong  and  thick,  as  is  the  case  with  the  elytra, 
or  wing-covers,  of  the  Coleoptera  (Beetles).  The 
chitinisation  of  the  epithelial  layer  is  not  confined  to 
the  surface,  and,  just  as  in  Echinoderms,  spicules  or 
plates  may  be  found  in  the  walls  of  the  digestive 
tract,  or  in  the  generative  glands  ;  so,  too,  chitin  may 
invade  the  stomodseal  and  proctodseal  portion  of  the 
alimentary  tract  of  Arthropods,  or  give  rise  to  a 
definite  series  of  internal  supporting  pieces,  the 
endosternites. 

These  chitinous  layers  are  not  formed  of  cells,  but, 
like  the  cuticle  of  a  protozoon,  are  shed  out  by  cells, 
which  they  invest  with  a  continuous  layer ;  the  layer 
is  often  seen  to  be  laminated,  or  made  up  of  a  number 
of  superimposed  secondary  layers,  laid  down  in  suc- 
cession. They  are  traversed  by  vertically-running  pore- 
canals,  and  are  often  strengthened,  especially  in  the 
Crustacea,  by  the  deposition  of  calcareous  salts.*  A 
firm  outer  coating  of  this  kind,  moulded  to  the  form 
of  the  body,  would  speedily  limit  the  growth  of  an 
Arthropod,  were  it  not  for  the  process  of  shedding,  or 
exuviation,  which  obtains  during  growth,  and  much 
more  frequently  in  young  and  rapidly-growing  forms 
than  in  those  which  have  attained  to  their  full  size. 

*  In  the  Crayfish  more  than  half  of  the  whole  weight  of  the 
exoskeleton  is  due  to  the  presence  of  calcareous  salts. 


Chap,  ix.]        SKELETON  OF  ARTHROPODA.  299 

In  this  exuviation,  or  ecdysis,  the  internal  chitinous 
and  calcareous  parts  are  as  much  affected  as  the 
external ;  when  it  is  completed  the  integument  of  the 
animal  is  for  a  few  days  soft  and  moist,  but  a  new 
exoskeleton  is  developed  with  comparative  rapidity, 
while  its  function  as  a  protective  organ  is  spoken  to 
by  the  temporary  timidity  of  animals,  which,  when 
armed,  are  bold,  and  ready  to  resent  attack. 

The  integument  is  smooth  in  the  lower  forms 
only ;  in  Peripatus,  as  in  the  larvse  of  insects, 
it  is  soft,  and  in  the  latter  it  is  sometimes  very  thin. 
It  is  generally  stouter  in  Myriopods  and  Arach- 
iiida,  and  its  degree  of  stoutness  varies  considerably 
in  Insects.  Among  the  Crustacea  it  is  compara- 
tively soft  in  Eiitomostraca,  except  where  valves 
are  developed ;  these  attain  to  their  greatest  hardness 
in  the  Cirripedia,  which  are  fixed  in  the  adult 
stages  of  their  lives,  and  are  therefore  unable  to 
escape  from  enemies  ;  they  are  stronger  and  more 
compact  in  the  sessile  Balanus  than  in  the  stalked 
Lepas ;  some  Cirripeds  have,  however,  lost  their  hard 
valves.  In  the  Malacostracous  Crustacea,  where  the 
carapace  has  more  definite  relations  than  in  such 
Entomostraca  as  Apus,  knobs,  ridges,  or  immovable 
spinous  processes,  all  of  which  are  defensive  in 
function,  are  very  commonly  developed.  Among 
the  Arachnida,  Limulus  is  remarkable  for  the  long 
caudal  spine-like  termination  (Fig.  124)  of  its  shield. 

Internal  hard  pieces,  which  appear  to  have  for  their 
chief  function  that  of  protecting  the  central  nervous 
system,  whereby  they  may  be  compared  to  the  verte- 
bral column  of  Vertebrates,  are  best  developed  in  the 
higher  Crustacea.  Though  topographically  and  func- 
tionally internal,  these  parts  are  morphologically  ex- 
ternal, and  they  share  in  the  general  moulting  of  the 
tegumentary  skeletal  parts  ;  these  ingrowths  are 
known  as  the  apodemes.  In  the  crayfish,  for 


300  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


example,  four  apodemes  are   well  developed  between 
every  two  thoracic  somites  \  the  inner  pair  unite  above 

and  below  so  as 
to  form  a  closed 
canal,  the  ster- 
nal canal ;  with 
these,  on  either 
side,  one  of  the 
outer  apodemes 
becomes  con- 
nected, and,  as 
it  also  becomes 
connected  with 
the  apodemes 
behind  it,  the 
several  parts  are 
united  into  a 
continuous  and 
substantial  in- 
ternal support- 
ing and  protec- 
tive mass,  which, 
in  addition  to  its 
other  functions, 
affords  attach- 
ment to  mus- 
cles. 

The  skeleton 
may  gain  in  pro- 
tective or  defen- 
sive power  by 
the  development 
of  spines,  pro- 
Fig.  124.— The  King-Crab  (Limulus  moluccanus).  cesses,  or  knobs, 

which  resist  the 

attacks  of  enemies,  or  are  able  to  passively  inflict  in- 
jury upon  them ;  sometimes,  indeed,  they  almost  come 


Chap. ix. j         SKELETON  OF  CRAYFISH.  301 

to  be  reckoned  among  active  agents  of  offence,  as  when 
they  are  developed  on  the  two  terminal  joints  of  the 
great  chelae  or  forceps,  the  last  of  which  is  movable  on 
the  last  but  one.  The  protective  power  of  the  hard 
exoskeleton  is,  inversely,  spoken  to  by  the  softness  of 
the  hinder  end  of  the  body  of  the  hermit-crab,  which 
lives  in  an  empty  snail-shell,  and  protrudes  only  the 
anterior  portion  of  its  body. 

The  larvse  of  various  insects  have  often  protective 
spines  or  warts ;  those  of  Crustacea,  in  the  Zoea-stage, 
have  more  or  less  long,  anterior,  dorsal,  and  lateral 
spines. 

The  several  parts  of  which  the  skeleton  is  made 
up  may  be  conveniently  studied  in  one  of  the  ab- 
dominal segments  of  a  crayfish.  We  here  see  that 
there  is  a  continuous  ring,  the  convex  dorsal  region 
(tergum)  of  which  is  continued  into  two  lateral 
(pleura.!)  regions,  which  hang  down  on  either  side  ; 
beneath  is  a  flattened  ventral  region  (sternum),  to 
which  are  articulated  two  jointed  appendages;  the 
piece  between  the  articulation  of  the  appendage  and 
the  pleuron  of  either  side,  is  known  as  the  epimerou. 
The  appendage,  when  completely  developed,  consists 
of  a  two-jointed  basal  protopodite,  with  which  are 
articulated  an  inner  and  an  outer  branch,,  which  are 
known  respectively  as  endopodite  and  exopodite ; 
with  these  a  third  piece,  epipodite,  is  sometimes  con- 
nected. In  the  crayfish  the  appendages  of  the  abdo- 
men are  either  flattened  to  serve  as  swimmerets,  or 
modified  to  act  as  accessory  reproductive  organs  (see 
page  496) ;  the  last  pair  of  appendages  are  greatly 
flattened  out,  and,  with  the  last  segment  (telson), 
which  is,  in  all  known  cases  but  that  of  Scyllarus, 
without  appendages,  forms  the  tail-fin.  The  four 
hindermost  pairs  of  thoracic  appendages  are  con- 
verted into  walking  limbs  by  a  considerable  in- 
crease in  the  size  and  strength  of  the  endopodite, 


302   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

which  constantly  consists  of  five  joints,  known  as  the 
ischio-,  mcso-,  carpo,  pro-,  and  dactylo-po- 
dit.cs.  The  pair  next  in  front  form  the  great  "  for- 
ceps "  or  chelae,  the  propodite  of  which  is  produced 
and  articulated  upon  the  dactylopodite,  so  as  to  form 
a  most  efficient  seizing  organ.  The  six  pairs  next  in 
front  form  the  Onathites,  the  modifications  of  which 
have  been  already  described  in  connection  with  the 
organs  of  digestion  (page  123).  The  two  most  ante- 
rior appendages  form  the  antennae  and  the  anten- 
miles ;  in  the  former  the  exopodite  forms  a  flattened 
squame,  and  the  endopodite  is  many -jointed  ;  in  the 
antenmile  both  endopodite  and  exopodite  consist  of  a 
number  of  joints.  By  some  authors  the  eye-stalks 
are  regarded  as  representing  the  protopodites  of  an 
appendage. 

The  paired  appendages  of  the  Arthropoda  take  on 
very  various  functions  in  different  groups,  and  vary 
considerably  in  number  ;  in  the  Phyllopoda,  the 
IVIalacostraca,  and  the  Myriopoda,  all  the  seg- 
ments of  the  body  bear  appendages;  in  the  Copepoda 
and  Arachnid  a  they  are  absent  from  the  hinder 
part  of  the  body ;  and  in  the  insects  (Hexapoda), 
there  are  but  three  pairs  of  definitely  constituted  ap- 
pendages behind  the  gnathites,  and  these  are  always 
attached  to  the  thorax. 

The  appendages  may  be  very  simple,  and  may  be 
nearly  all  similar  in  function,  as  in  Peripatus, 
where  the  incompletely  jointed  appendages,  provided  at 
their  free  end  with  their  two-hooked  claw,  are  nearly 
all  ambulatory  in  function  ;  one  pair  alone  forming 
gnathites,  one  oral  papilla?,  and  the  last  of  all  the  anal 
papillae.  In  the  Myriopoda,  all  behind  the  gnathites 
are  ambulatory  in  function  ;  in  the  Branchiopoda  they 
form  branchial  swimmerets,  and,  as  in  other  Entomos- 
traca,  there  are  never  more  than  three  pairs  of 
gnathites  ;  in  the  Copepoda  and  Ostracoda  the  second 


Chap,  ix.]  APPENDAGES  OF  INSECTS.  303 

pair  of  antennae  retain  the  natatory  function  which 
they  have  in  the  Nauplius  stage.  In  the  Cirripedia 
the  six  pairs  of  appendages  behind  the  gnathites  have 
the  exopodite  and  endopodite  consisting  of  a  large 
number  of  joints,  and  they  form  the  filamentous  cirri 
which  are  so  characteristic  of  these  animals.  In  the 
Arachnid  a  there  are  no  appendages  in  front  of  the 
head,  antennas  being  absent.  In  Peripatus  the  an- 
tennae do  riot  belong  to  the  series  of  ventral  appendages 
of  the  segments  of  the  body.  In  Myriopods  and  In- 
sects there  is  a  single  pair  of  antennae.  In  the 
Arachnida  the  basal  parts  of  the  circum-oral  appen- 
dages alone  take  part  in  the  service  of  the  mouth  ; 
the  most  anterior  pair  are  pincer-shaped  at  their  free 
end  (clidicerse) ;  in  some  Myriopods  one  of  the 
anterior  pairs  of  appendages  become  poison- claws,  as 
are  the  chelicerae  in  spiders.  In  the  parasitic  Peii- 
tastomida  all  signs  of  appendages  are  reduced  to  two 
pairs  of  curved  hooks  in  the  region  of  the  mouth. 

The  Hexapoda  have  only  three  pairs  of  gnathites 
(see  page  128) ;  and  these,  as  has  been  already  pointed 
out,  present  the  most  diverse  modifications  in  different 
orders.  The  legs  are  almost  always  well  developed  on 
the  three  segments  of  the  thorax,  and  are  typically 
five-jointed  ;  the  most  proximal  is  known  as  the  coxa- 
and  this  is  succeeded  by  the  ordinarily  smaller 
troclia liter,  by  the  longer  femur,  and  the  still  longer 
tibia ;  the  last  .joint  or  tarsus  consists  of  several 
pieces,  the  most  distal  of  which,  or  that  farthest  from 
the  axis  of  the  body,  carries  a  pair  of  claws. 

The  number  of  legs  (six)  must  be  a  great  me- 
chanical advantage  to  an  insect,  for  three  supports  are 
necessary  to  maintain  a  stable  equilibrium.  They  may 
become  adapted  to  very  various  modes  of  progression 
through  earth  or  air.  In  the  digging  forms,  such  as 
Gryllotalpa  (the  mole-cricket),  the  tibiae  of  the  first 
pair  of  legs  are  flattened,  triangular,  and  toothed, 


304  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

and  are  supplied  with  well-developed  muscles.  In 
the  aquatic  forms,  such  as  the  water-beetles  (Dyticus), 
the  coxse  of  the  third  pair  of  legs  are  flattened  and 
oar-like.  Such  as  float  on  the  surface  of  the  water 
have  the  contained  air  tubes  enlarged  to  serve  as  float 
bladders,  or  the  legs  are  greatly  elongated  so  as  to 
extend  over  a  large  surface  of  water.  In  climbing 
insects  the  claws  may  be  cleft  or  pectinated  so  as  to 
enable  them  to  hold  on  to  small  objects ;  or  an  at- 
taching lobule  may  be  developed  between  the  claws. 

The  tergal  portions  of  several  successive  seg- 
ments may  unite  with  one  another,  and  thus  give 
greater  firmness  to  the  dorsal  surface ;  this  process 
may  result  in  the  formation  of  a  free-projecting  shell, 
as  in  Apus,  or  this  shell  may  become  divisible  into 
two  valves,  as  in  the  Ostracoda,  or  the  fusion  may 
extend  far  back,  as  in  the  crayfish,  or  the  scorpion, 
where  we  have  the  so-called  cephalo thoracic  cara- 
pace ;  in  Limulus  the  sides  of  the  carapace  are  pro- 
duced, and  we  get  the  well-known  large  shield  of 
these  animals ;  the  same  phenomenon  in  the  crayfish 
or  the  lobster  results  in  the  formation  of  a  special 
wall  for  the  branchial  chamber. 

The  most  remarkable  modifications  are  exhibited 
by  the  Cirripedia,  where  the  exoskeleton  is  ordi- 
narily in  the  form  of  calcified  valves,  two  on  either 
side  of  the  body,  and  in  Lepas  with  a  dorsal  median 
piece  ;  in  Balanus  these  valves  are  withdrawn  into 
a  special  shell. 

In  the  Mollusca  the  characteristic  organ  of  sup- 
port and  defence  is  an  external  calcareous  shell, 
which  is  formed  by  the  mantle,  and  which,  when 
aided  by  the  operculum,  which  is  developed  on  the 
base  of  the  foot,  becomes  so  completely  an  organ  of 
protection  that  many  snails  hibernate  in  their  closed 
shell  ;  the  tenant  of  an  exotic  shell  has  been 
bought,  sold,  and  exhibited  in  a  museum  for  the  space 


Chap,  ix.]  STRUCTURE  OF  SHELLS.  305 

of  four  years  before  giving  any  signs  of  vitality  (Helix 
desertorum).  The  shell  is  sometimes,  however, 
merely  chitinous  and  internal,  as  in  the  slug  or  the 
squid,  or  it  is,  in  earlier  stages,  rudimentary,  and  in 
adult  life  completely  lost  (Nudibraiichs).  In  this 
phylum  there  is  no  arrangement  comparable  to  the 
exuviation  which  obtains  among  the  Arthropoda.  The 
dependence  of  these  calcareous  shells  on  the  nature  of 
their  surroundings  is  admirably  spoken  to  by  such 
facts  as  the  absence  of  shelled  forms  from  such  dis- 
tricts as  the  Lizard,  or  parts  of  Asia  Minor,  where 
there  is  no  lime  (Forbes)  ;  while,  on  the  other  hand, 
the  influence  of  the  animal  itself,  and  of  other  condi- 
tions, is  expressed  by  the  greater  density  of  the  shells 
of  the  mussel  of  the  m6untam  streams  of  Westmore- 
land, as  compared  with  the  thinner  shells  of  the  Isis, 
which,  at  Oxford,  is  richer  in  salts  of  lime  than  the 
just  mentioned  more  northern  waters  (Rolleston). 

Shells  vary  considerably  in  texture  and  struc- 
ture ;  many  are,  as  is  well  known,  pearly  within ; 
these  nacreous  shells  owe  their  characteristic 
appearance  to  the  alternate  deposition  of  layers  of 
thin  membrane  and  of  carbonate  of  lime,  and  to  the 
irregular  deposition  of  the  thin  layers,  which,  by 
slightly  overlapping  one  another,  diffract  the  light  and 
so  give  rise  to  the  iridescence  of  the  internal  surface. 
This  explanation  of  the  physics  of  the  appearance  of 
the  shell  may  be  applied  also  to  the  "  pearls," 
whether  formed  naturally  by  the  deposition  of  thin 
layers  around  an  organic  or  inorganic  nucleus,  or 
artificially  by  the  introduction  of  foreign  material. 
Where  the  lustre  is  duller  than  in  nacreous  shells,  the 
hard  structures  are  said  to  be  porcellaiious;  in 
Pinna  and  others  the  shell  is  said  to  be  fibrous,  owing 
to  the  fact  that  the  separate  parts  of  each  layer 
correspond  to  one  another,  and  fracture,  therefore, 
results  in  a  number  of  vertical  pieces.  The  laminated 
u— 16 


306  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

arrangement,  or  disposition  of  the  la}^ers  of  the  shell 
in  horizontal  planes,  is  well  seen  in  the  oyster.  Where 
the  possessor  of  the  shell  is  in  the  habit  of  floating 
(lanthina,  Argonauta),  the  "  shell  "  is  comparatively 
thin,  and  of  low  specific  gravity. 

In  the  most  primitive  Mollusca  the  shell  is  merely 
spicular ;  in  Neomenia  the  spicula  are  arranged  in  a 
single  layer  in  the  outer  part  of  the  integument,  and 
protrude  their  pointed  ends  from  its  surface ;  in 
Proneomenia  the  spicules  are  set  in  several  layers, 
placed  in  a  chitinous  interspicular  substance.  In  both 
cases  the  spicules  consist  of  carbonate  of  lime.  A 
higher  stage  is  found  in  the  Chitons,  where  the  shell 
consists  of  eight  plates  which  lie  on  the  back  of  the 
animal,  and  have  an  arrangement  which  we  can 
hardly  resist  from  regarding  as  metameric.  In  the 
Lamellibrancliiata  the  shell  consists  of  two 
valves,  which  lie  to  the  right  and  left  of  the  animal, 
and  are  connected  with  one  another  by  a  chitinous 
ligament,  which  runs  along  the  middle  line  of  the  back ; 
this  ligament  is  elastic,  and  in  consequence  of  this  pro- 
perty, the  animal  in  a  state  of  repose  is  able  to  keep 
its  two  valves  slightly  separated  without  incurring  any 
expense  of  muscular  activity.  At  the  approach  of 
danger  the  valves  can  be  approximated  so  as  to  pro- 
tect their  possessor,  by  the  contraction  of  two  (fresh- 
water mussel)  or  one  (oyster)  pair  of  adductor  muscles. 
In  the  higher  Cepiialophora  the  shell  is  always 
single,  whence  they  are  often  distinguished  by  the  name 
of  univalves  from  the  Lamellibranchs,  which  are  the 
bivalves.  This  shell  may  form  a  single  conical  cup, 
as  in  the  limpet  (Patella),  or  it  may  be  slightly  coiled, 
as  in  the  cowry,  or  it  may  be  greatly  coiled  and 
consist  of  a  number  of  chambers,  as  in  the  nautilus. 
It  is  certain  that  many  conchologists  go  too  far  in  the 
trust  that  they  put  in  the  characters  of  the  shell,  but 
it  remains  as  a  matter  of  fact  that  in  the  great 


Chap.  IX.] 


STRUCTURE  OF  SHELLS. 


307 


majority  of  cases  the  exact  systematic  position  of  a 
mollusc  may  be  determined  by  the  shell  alone,  so 
marked  are  the 
differences,  and 
so  deep  -  seated 
the  essential  cha- 
racteristics. Geo- 
logists believe 
that  there  is  no 
evidence  more 
worthy  of  con- 
fidence than  that 
which  is  afforded 
them  by  the 
shells  of  any 
given  deposit. 

The  shell, 
which  owes  its 
growth  to  the 
activity  of  the 
outer  cell-layers 
of  the  mantle, 
commences  as  a 
pit  or  invagina- 
tion  of  the  outer 
layer  on  the  abo- 
ral  surface  of  the 
larva  ;  this  pit  is 
the  shell  gland 
of  Lankester,  and 
it  secretes  a  vis- 
cid body  which 
hardens  on  con- 
tact with  water ;  this  hardened  substance  is  the  earliest 
rudiment  of  the  shell,  and  even  in  the  bivalved  forms 
it  is  at  first  a  single  saddle-shaped  plate,  which  only 
later  becomes  divided  into  two  bilateral  halves. 


Fig.  125.— Shell  of  Triton,  to  explain  the  terms 
used  in  the  descriptions  of  Shells. 


The  -shell  is  fusiform,  in  shape  :  its  apex  (A)  is  niam- 
niillated  ;  it  is  made  up  of  whorls  (w),  separated 
by  sutures  (su) ;  bw,  body  whorls  There  is  an  internal 


millated  ;  it  is  made  up  of  whorls  (w),  separated 
by  sutures  (su) ;  bw,  body  whorls  There  is  an  internal 
axis  or'columella  (i),  an  outer  Up  (o),  an  aperture  (a), 
and  an  anterior  (etc)  and  posterior  (pc)  canal.  (After 
Woodward.) 


308  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY, 

In  many  cases  lines  of  growth  can  be  made  out, 
and  the  shell  may  continue  to  increase  in  size  for  so 
long  a  time  that  a  single  valve  of  a  Tridacna  will  be 
found  to  weigh  a  hundred  and  fifty-six  pounds. 

In  Nautilus  the  shell  consists  of  a  number  of 
chambers  (Fig.  126),  each  of  which  is  larger  than  that 
which  precedes  it,  and  is  formed  by  the  animal  as  it 
increases  in  size ;  each  of  these  chambers  is  separated 


Fig.  126.— A  Section  of  tbe  Shell  of  the  Pearly  Nautilus,  showing  the 
successive  Chambers  occupied  by  the  Animal. 

a,  Mantle;  b,  dorsal  fold;  g,  muscle;  ii,  siphuncle;  fc,  funnel;  n,  hood;p,  ten- 
tacies ;  «,  eye;  x,  septa  ;  z,  last  chamber. 

from  one  another  by  a  septum  (x),  and  the  whole 
mass  is  spirally  coiled  on  itself.  The  chambers  are 
connected  with  one  another  by  a  tube  or  siphuncle 

(ii),  the  presence  of  which  has  given  rise  to  the  belief 
that  the  whole  series  forms  a  kind  of  float  by  means 
of  which  the  animal  is  enabled  to  remain  at  will  on  the 
surface  of  the  water ;  such  definite  observations, 
however,  as  have  been  made  on  living  specimens, 
and  the  fact  that  though  (like  the  shells  of  Spirula) 
the  shells  are  common  enough,  but  the  animals  very 
rare,  lead  us  rather  to  believe  that  Nautilus  is  essen- 
tially a  dweller  at  the  bottom  of  the  ocean.  Here, 


chap,  ix.]  FORMS  OF  SHELLS.  309 

then,  we  have  another  example  of  the  danger  of 
arguing  d,  priori  as  to  function  from  structure. 
Physiology,  like  other  branches  of  science,  must  pro- 
ceed rather  by  observation  and  ct,  posteriori  argu- 
ments. 

Nautilus  is  the  only  existing  tetrabranchiate 
Cephalopod,  but  to  that  division  belonged  a  number 
of  extinct  forms,  whose  shells  are  found  fossil ;  such 
are  the  Ammonites,  with  shells  like  those  of  the 
Nautilus,  Gyroceras  with  discoidal,  Trochoceras  with 
spiral,  and  Baculites  with  straight  shells. 

Among  the  Diforanchiata,  Spirula  (Fig.  127;  A), 
whose  body  is  so  rare  and  whose  shell  so  common, 
alone  has  the  shell  coiled  and  divided  into  chambers ; 
it  is  not,  however,  an  external  shell,  like  that  of  the 
nautilus,  but  is  internal.  In  the  fossil  Belemmite 
(Fig.  127;  B)  the  proximal  end  of  the  shell  (phrag- 
mocone)  was  divided  into  separate  chambers,  which 
were  connected  by  a  siphuncle.  The  distal  end  of  the 
shell  is  dart-shaped  and  solid,  and  forms  the  so-called 
giiard.  In  Sepia  the  shell  is  calcareous,  straight, 
flattened  out  for  the  greater  part  of  its  length,  with 
the  apex  only  incompletely  chambered. 

In  the  squids,  such  as  Loligo  (Fig.  127 ;  c),  the  shell, 
now  ordinarily  known  as  the  pen,  is  merely  horny, 
and  consists  of  a  shaft  with  two  wings;  in  the  Octopus 
the  shell  is  lost ;  its  ally  Argonauta  (Fig.  1 27 ;  D) 
fashions  for  itself  a  shell  which  both  morphologically 
and  physiologically  is  a  different  structure  from  those 
we  have  hitherto  been  considering,  for  it  is  formed 
by  a  pair  of  the  arms  and  not  by  the  mantle  cells,  and 
is  confined  to  the  female,  where  it  serves  to  carry  the 
fertilised  ova.  A  structure,  the  origin  of  which  is 
unknown,  but  the  function  of  which  is  likewise 
incubatory,  is  that  which  is  known  as  the  float  of  the 
Gastropod  lanthina. 

The  operculum  is  formed  by  the  foot,  is  horny, 


3io  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


and  sometimes  impregnated  with  calcareous  salts ;  it 
is  very  generally  distributed  among  the  Gastropoda, 


Fier.  127.— A,  Section  of  Spirula  australis,  to  show  the  internal  shell ;  B, 
Belemnite  restored  ;  c,  Pen  of  the  Loligo ;  D,  Argonaut  a  in  Natural 
Position.  (From  Woodward.) 

and  may  completely  fit  the  mouth  of  the  shell,  as  in 
the  common  snail,  or  close  part  only  of  it,  as  in 
Strombus;  or,  as  in  Dolium  and  other  large-mouthed 


Chap,  ix.]  FORMS  OF  SHELLS.  311 

shells,  it  may  be  rudimentary,  or  absent.  The  operculum 
is  not  represented  in  the  Lamellibranchs.  In  a  few  cases 
(e.g.  the  Lamellibranchiate  Aspergillum)  the  protective 
function  is  not  assumed  by  the  shell,  which  may  be 
quite  small,  but  by  the  deposit  of  calcareous  matter 
on  the  siphon- shaped  prolongations  of  the  mantle. 
(See  page  80.) 

Among  the  Gastropoda  the  shell  is  often  greatly 
reduced,  as  in  the  slug,  or  completely  lost,  as  in  Doris 
and  other  Nudibranehs,  where  the  integument  is, 
however,  richly  supplied  with  calcareous  spicules,  and 
in  Oncidium,  where  the  integument  is  thick  and 
leathery.  One  large  division  of  Pteropods  are 
without  any  shell,  and  in  the  thecosomatous  forms  it 
is  always  thin  and  glassy. 

The  internal  skeleton  of  Gastropods  consists 
merely  of  one  or  two  pairs  of  cartilaginous  plates, 
which  are  found  in  the  region  of  the  pharynx.  It  is 
better  developed  in  the  Cephalopoda,  where  it  is 
represented  by  a  median  cephalic  cartilage,  which 
is  pierced  in  the  middle  line  by  the  resophagus, 
and  is  produced  on  either  side  into  plate-like  supports 
for  the  eye  and  ear;  in  some  cases  the  orbit  is 
completely  surrounded  by  cartilaginous  pieces.  In 
the  Dibranchiata  the  muscles  which  move  the  fins, 
when  those  organs  are  developed,  are  inserted  into 
special  cartilages,  and  other  cartilaginous  pieces  are 
more  irregularly  developed  at  the  base  of  the  funnel, 
on  the  dorsal  surface,  or  in  the  neck. 

The  Brachiopoda  have  a  hard  external  shell, 
which,  though  it  consists  of  two  valves,  is  not  to  be 
compared  with  that  of  the  lamellibranchiate  mollusc, 
for  their  valves  are  not  right  and  left,  but  dorsal  and 
ventral  in  position ;  they  may  be  subequal,  as  in 
Lingula,  or  the  ventral  may  be  prolonged  into  the  beak- 
shaped  free  end,  which  has  gained  for  these  animals 
their  familiar  name  of  "  lamp-shell ; "  in  the  latter 


312  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  two  valves  are  hinged  on  one  another  (Testi- 
cardines  or  Articulata).  The  dorsal  valve  may 
give  off  hard  processes,  which  project  into  the  cavity 
between  the  two  valves,  and  which  has  a  spiral  or 
looped  form  (Fig.  128) ;  they  serve  as  a  means  of 
support  for  the  highly-developed  "  arms "  of  the 


Fig.  128.— Dorsal  Valves  of  various  Brachiopods,  seen  from  within,  to 
show  the  loop  (1).  A,  Terebratula ;  B,  Terebratulina ;  c,  Terebratella ; 
D,  Bouchardia;  E,  Megerlia ;  F,  Argiopej  h,  Hinge;  I,  loop;  s, 
septum.  (After  Davidson.) 

Brachiopoda.  In  some  (e.g.  Terebratula)  the  sub- 
stance of  the  shell  is  traversed  by  tubular  prolonga- 
tions of  the  contained  mantle.  Scattered  calcareous 
spicules  are  to  be  found  in  the  integument. 

Among  the  Chordata,  a  well-developed  skeleton 
appears  only  in  the  Verteforata,  where  it  attains  to 
considerable  complexity ;  the  more  characteristic  is 
internal,  but  an  external  skeleton  is  sometimes  also 
present. 

All  Vertebrate  at  some  period  of  their  lives,  the 


Chap.  IX.]  NOTOCHORD.  313 

Ceplialocliordata  throughout  their  life,  and  the 
Urochordata  either  permanently,  temporarily,  or 
never,  have  an  internal  organ  of  support  in  the  form 
of  a  rod,  the  so-called  notoctiord  or  dorsal  rod, 
which  lies  just  beneath  the  central  nervous  system ; 
the  substance  of  which  this  rod  is  formed  appears  to  be 
allied  to  cartilage  ;  in  Amphioxus  it  becomes  much 
more  complex  in  structure  than  in  other  Chordata,  so 
that  we  have  here  an  example  of  how  an  organ  of  an 
animal  which  remains  at  a  certain  grade  in  the  scale 
of  development,  becomes  more  complex  and  elaborate 
under  its  conditions  than  does  the  same  organ  in  a 
"  higher  animal,"  where  it  serves  only  a  temporary 
purpose.  In  the  Urochordata  the  notochord  is  only 
found  in  the  tail ;  it  either  is  persistent,  or  is  aborted 
when  the  free-swimming  larva  settles  down  to  a  fixed 
mode  of  life,  or  it  is  never  developed  at  all ;  as  in  the 
rest  of  the  division,  it  has  a  stout  continuous  sheath, 
and,  as  it  is  elastic,  it  brings  the  tail  back  into  posi- 
tion when  the  organ  has  been  bent  by  the  muscles 
attached  to  it.  In  the  Urochordata,  therefore,  the 
notochord,  when  present,  may  be  regarded  as  having 
also  a  locomotor  function. 

Amphioxus  has  no  external  skeleton,  nor  have 
those  Urochords  that  are  tailed  throughout  life  j  in 
the  rest,  the  "  outer  mantle,"  or  test,  may  become 
very  strong  and  rigid,  so  as  to  form  a  complete 
organ  of  protection;  it  is  remarkable  for  containing 
cellulose,  a  starchy  compound  which,  so  common  in 
vegetable  organisms,  is  only  known  among  animals 
in  the  Tunicata  and  the  protozoic  Cilio-nagellata ; 
scattered  calcareous  spicules  are  not  unfrequently 
deposited  in  the  cells  of  the  mantle,  but  never  form 
a  continuous  layer.  The  tailed  forms,  such  as  Ap- 
pendicularia,  are  able  to  rapidly  secrete  an  invest- 
ment, the  so-called  "  house ; "  in  this,  however,  they 
do  not  dwell  permanently,  but  are  described  as 


314  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

leaving    one    after  a  few  hours,  and    as    forming    a 
fresh  one,  when  they  again  settle  down. 

In  the  greater  number  of  the  Vertebrata  the 
notochord  is  very  profoundly  modified  in  character, 
and  in  the  higher  forms  it  disappears  altogether  from 
the  adult,  its  place  being  taken  by  the  jointed 
vertebral  column.  In  the  lowest  stages,  such 
as  are  found  in  the  Cyelostomata,  Chimrera 
among  the  Elasrnobranchs,  and  the  Dipnoi,  the  noto- 
chord remains  unconstricted,  but  cartilaginous  or 
calcareous  deposits  become  aggregated  around  it, 
while  above,  and  sometimes  also  below  it,  there 
appear  arches  of  cartilage,  which  protect  the  over- 
lying nerve-cord  and  the  subjacent  blood-vessel. 

In  more  complete  Vertebrae  it  is  possible  to 
distinguish  a  basal  portion,  or  centrum,  which 
grows  round  the  notochord,  from  the  overlying 
neural  and  the  underlying  haemal  arches,  which 
enter  into  more  or  less  close  union  with  it.  In  many 
Fishes  the  notochord  remains  well  developed  between 
the  separate  vertebral  centra,  and  these  are,  in  the 
simplest  cases,  excavated  both  in  front  and  behind, 
whence  they  are  known  as  amphicoelous  ver- 
tebrae ;  in  better  developed  forms  a  smaller  amount 
of  notochord  is  persistent,  and  we  then  get  either 
procoelous  or  opisthocoelous  vertebrae,  accord- 
ing as  the  excavation  is  on  the  anterior  or  the  pos- 
terior face  of  the  centrum  ;  sometimes,  as  in  the  frog, 
the  invasion  of  the  notochord  by  cartilage  or  bone  is 
never  complete,  and  in  such  cases  a  cross-section 
of  the  centrum  of  the  vertebra  reveals  the  presence 
of  a  central  notochord  (Fig.  129;  ch).  In  the  frog, 
as  in  the  Amphibia  generally,  the  neural  arch 
and  the  centrum  become  firmly  connected  with  one 
another,  and  from  the  centrum  there  are  given  off 
horizontal  pieces  of  bone,  which  form  the  so-called 
transverse  processes.  The  separate  vertebrae 


Chap.  IX.  J 


VERTEBRA. 


3T5 


are  articulated  with  one  another  by  means  of  pro- 
cesses directed  forwards  and  backwards,  the  zyga- 
pophyses,  as  these  articulating  outgrowths  are  called. 

The  shape  of  the  faces  of  the  centra  of  the  ver- 
tebrae varies  greatly,  not  only  in  different  forms  of 
the  Saiiropsida,  but  even  in  different  parts  of  the 
vertebral  column  of  the  same  individual.  In  the 
Ophidia  these  differences  are  seen  to  be  associated 
with  their  mode  of  life  ;  the  anterior  face  is  deeply 
hollowed,  and  the  posterior 
rounded  and  convex ;  the 
convexity  fitting  into  the 
concavity  of  the  next  suc- 
ceeding vertebra,  and  being 
capable  of  rotation  within  it ; 
in  addition  to  this,  the  faces 
of  the  neural  arch  are  modi- 
fied, the  anterior  being  pro- 
duced into  two  wedge-shaped 
processes  (zygosphenes), 
which  fit  into  corresponding 
depressions  (zygantra)  on 
the  hinder  face  of  the  arch,  and 
thereby  form  a  kind  of  peg-and«-socket  joint  (Fig.  130). 
Vertebrae  of  this  kind  are  found  also  in  the  lacertilian 
Iguanse,  who  are  known  to  swim  by  the  movements 
of  their  tails. 

In  Hatteriaj  the  Geckos,  and  some  fossil  lizards, 
the  notochord  is  persistent  between  the  vertebrae, 
the  centra  of  which  a,re,  therefore,  amphiccelous. 
Among  the  Crocodiles  a  progressive  loss  of  the  inter- 
vertebral  portion  of  the  notochord  may  be  made  out, 
such  forms  as  lived  before  the  period  of  the  Chalk 
having  amphiccelous,  while  cretaceous  and  post-cre- 
taceous crocodiles  have  proccelous,  vertebrae.  Among 
Birds,  the  fossil  Arcbaeopteryx  and  Ichthyornis 
appear  to  have  had  amphiccelous  vertebrae,  but  in  all 


Pig.  129.— Section  through  the 
Vertebra  of  a  Frog,  magni- 
fied. 

ch,  Notochord;  chs,  its  sheath; 
o,c  different  kinds  of  bone. 
(After  Ecker.) 


316  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

recent  forms  the  centra  of  the  vertebrae  are  exceed- 
ingly well  ossified  ;  they  have,  especially  in  the  region 
of  the  neck,  an  exceedingly  characteristic  form  of 
surface,  for  they  are  saddle-shaped,  being  convex  from 
side  to  side,  and  concave  from  above  downwards  on 
their  anterior  face  ;  as  exactly  the  opposite  arrange- 
ment obtains  on  the  posterior  surface,  it  is  obvious 
that  the  vertebrae  are  able  to  move  on  one  another, 
and  the  neck  capable  of  that  mobility  which  is  so 
notable  and  useful  a  possession  of  the  bird,  whose 

ayi 


Fig.  130.— Anterior  and  Posterior  Surfaces  of  the  Vertebrae  of  a  Snake, 
showing  the  form  of  the  Centra,  and  the  Zygosphenes  (zys),  and 
Zygantra  (zyt). 


anterior  appendages  are  of  no  use  for  seizing  food 
or  other  objects.  An  exactly  analogous  arrange- 
ment to  this  may  be  observed  among  the  Ophiu- 
roidea  ;  in  the  larger  number  of  these  brittle  stars  the 
several  ossicles  have  a  certain  power  of  movement  on 
one  another,  but  this  is  limited  by  the  development  of 
processes  and  pits  analogous  to  the  zygosphene  and 
zygantra  of  the  Ophidian  vertebrae.  In  such  Ophiu- 
roids,  however,  as  are,  like  Astroschema,  capable  of 
twisting  or  twining  their  arms  round  a  straight 
Gorgonian,  the  saddle -shaped  faces  are  well  developed, 
but  the  limiting  pits  and  processes  are  absent. 

In  the  Mammalia  the  faces  of  the  centra  are 
often  nearly  plane,  and  from  the  intervening  cartilage 
there  is  developed  (except  in  the  Prototheria,  where 


Chap.  IX.]  VER  TEBRM.  3 1 7 

there  are  only  occasional  rudiments  in  the  tail,  and, 
much  more  remarkably,  in  the  Sirenia)  disc-like  plates 
of  bone,  the  so-called  epiphyses,  which,  on  the 
arrival  of  maturity,  fuse  with  the  centra,  and  obscure 
the  line  of  union  (neuro-central  suture)  between  the 
centra  and  the  neural  arches. 

On  the  dorsal  surface  of  these  arches  a  spinous 
process  (neural  spine)  is  often  developed,  and  from 
these  muscles  may  have  their  origin  ;  forming  only  a 
feeble  ridge  in  the  Amphibia,  though  prominent  in 
many  fishes,  they  are  often  large  in  the  Sauropsida, 
and  are  of  considerable  importance  in  many  Mammals. 

It  is  in  the  highest  forms  that  we  can  best  distin- 
guish the  several  so-called  regions  of  the  vertebral 
column.  In  such  a  Mammal  as  the  rabbit,  it  is,  for 
example,  possible  to  make  out  (1)  a  cervical  region, 
in  which  the  laterally  placed  ribs  are  never  more 
than  rudimentary;  the  vertebrae  of  this  region, 
whether  the  neck  is  as  long  as  in  the  giraffe,  or  as 
short  as  in  the  porpoise,  is  always  composed  of  seven 
vertebrae,  with  the  exception  of  the  three-toed  sloths 
(Bradypus)  which  have  nine;  of  another  Edentate 
(Manis),  which  has  sometimes  eight  (W.  K.  Parker)  ; 
and  of  one  two-toed  sloth  (Chokepus  hoffmani),  and 
the  Manatee,  which  have  six.  (2)  A  thoracic 
region,  with  which  are  connected  ribs  that  are  movably 
articulated  with  them,  and  some  of  which  join  the  • 
veiitrally  placed  sternum,  and  so  form  a  kind  of  pro- 
tecting cage  for  the  thoracic  viscera,  and  points  of 
attachment  for  the  important  costal  muscles.  (3)  A 
lumbar  region,  where  the  ribs  are  not  movably 
articulated,  but,  being  shorter,  leave  space  for  the 
coils  of  the  intestine  and  the  distension  of  the  abdo- 
men which  occurs  in  gravid  females  of  this  group. 
(4)  A  sacral  region,  the  definition  of  which  is  sur- 
rounded with  considerable  difficulties,  but  which  is, 
perhaps,  best  defined,  with  Gegenbaur  and  A.  Milne- 


Chap.  IX.]  VEK  TEBRAL   COL  UMN.  319 

Edwards,  as  the  region  in  which  the  vertebrae  have 
additional  (pleurapophysial)  centres  of  ossification 
for  the  attachment  of  the  ilium  (Fig.  132),  and 
which  is  denned  posteriorly  by  the  point  of  inser- 
tion of  the  ischio-sacral  ligament.  There  are  ordi- 
narily two  true  sacral  vertebrae,  but  with  them  there 
often  become  connected  some  of  the  (5)  caudal  or 
tail  vertebrae  ;  the  whole  fusing  to  form  a  single  bone 
of  great  strength.  (See  page  321.)  The  caudal 
vertebrae  vary  greatly  in  number,  according  to  the 
length  of  the  tail.  The 
greatest  known  number 
among  mammals  is  found 
in  the  insectivorous 
Microgale  longicauda, 
which  may  have  as  many 
as  forty-eight ;  Manis  has 
forty  -  six.  Connected 

with  and  intermediate  to      Fig.  132.~ Anterior  Surface  of  First 

the   several   caudal  ver-  Sacral  Vertebra  of  Mat3' 

.  .         ,  ,       c,  Centrum  ;  na,  neural  arch;  p,  pleura- 

tebrae     are      V  -  shaped  pophytta. 

(chevron)  bones,  which 

protect  the  vessels  of  the  tail,   and  afford  a  larger 
surface  of  attachment  for  the  muscles. 

The  neural  spines  and  the  transverse  processes 
vary  very  considerably  in  length  and  size,  according 
to  the  functions  and  size  of  the  muscles  attached 
to  them  ;  the  transverse  processes,  for  example,  being 
long  in  the  lumbar  region  of  active  jumping  forms, 
such  as  the  hare' or  the  bandicoot. 

The  first  vertebra,  which  in  man  supports  the 
head,  has  been  on  that  account  called  the  atlas,  while 
the  second,  on  which  the  atlas  moves,  is  distinguished 
as  the  axis ;  the  centrum  of  the  atlas  is  remarkable 
for  either  fusing  with  that  of  the  axis  to  form  the 
odontoid  process,  or,  as  in  the  Monotremata  and 
many  Reptiles,  it  persists  as  an  independent  bony  piece. 


320  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  importance  of  the  vertebral  column  is  well 
illustrated  by  the  arrangements  which  obtain  in  the 


Fig.  133.— Skeleton  and  Carapace  of  the  Loggerheaded  Turtle 
(from  below). 


Mammalia.  We  find  that  not  only  does  the  bony  tube 
afford  a  complete  defence  for  the  enclosed  spinal  cord, 
and  that  the  several  parts  of  which  it  is  composed  can 
be  bent  on  one  another,  but  that  it  is  elastic  in  virtue 


chap,  ix.]  VERTEBRAL  COLUMN.  321 

of  the  cartilaginous  discs  that  lie  between  the  several 
vertebrae,  and,  in  the  more  erect  forms,  by  its 
sigmoid  curvature  ;  the  bony  outgrowths  of  the 
several  parts  may  be  elongated  or  broadened  to  serve 
as  larger  areas  of  attachment  for  the  muscles. 

The  number  of  cervical  vertebrae  among  the 
Sauropsida  varies  with  the  length  of  the  neck ; 
the  swan,  for  example,  having  as  many  as  twenty- 
five.*  In  all  these  hand-less  Vertebrates  the  neck 
is  of  great  mobility  ;  freedom  of  movement  to 
the  vertebrae  on  one  another  being  allowed  by  the 
already  described  saddle-shaped  form  of  their  centra. 
In  the  Chelonia  also,  where  much  of  the  body  is  in- 
vested in  the  firm  carapace,  the  neck  is  very  flexible, 
and  the  shape  of  the  centra  varies  greatly  not  only  in 
different  species,  but  in  the  different  cervical  vertebrae 
of  the  same  animal ;  the  neural  spines  are  never  well 
developed,  and  the  head  can  be  retracted  or  protruded. 
The  succeeding  vertebras  in  the  Chelonia  have  flattened 
centra,  but  most  are  more  remarkable  for  the  possession 
of  a  broad  plate  of  bone,  which  is  connected  with  the 
apex  of  the  neural  arch,  and  forms  one  of  the  median 
"  neural  plates  "  of  the  exoskeletal  carapace  •  the  ver- 
tebrae of  the  tail  can  be  moved  on  one  another,  and 
are,  like  most  of  the  vertebrae  of  most  Reptiles,  procce- 
lous.  The  most  important  exceptions  to  this  law 
have  been  already  noted. 

While  in  the  Amphibia  only  one  vertebra  enters 
into  relation  with  the  ilium,  or  can  be  spoken  of  as 
sacral,  there  appear  to  be  two  true  sacral  vertebrae  in 
the  Saiiropsida.  In  Birds,  where  the  whole  support 
of  the  body  falls  on  the  hind  limbs,  a  number  of  pre- 
sacral  and  post-sacral  vertebrae  fuse  with  the  true 
sacrals,  to  form  a  firm  mass  of  attachment  and  sup- 
port. Where  the  bird  has,  like  the  ostrich,  to  depend 

*  Some  of  the  extinct  tlesiosauria  had  more  than  forty  cervical 
vertebrae. 

v— 16 


322    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

entirely  oh  its  legs,  not  only  for  support,  but  also  for 
means  of  locomotion,  the  number  of  vertebrae  that  so 
fuse  together  may  be  greater  than  twenty.  Chevron 
bones  are  sometimes,  as  in  lizards  and  crocodiles,  de- 
veloped in  the  caudal  region,  the  length  of  which 
varies  considerably  among  reptiles ;  but  in  flying 
birds,  where  a  tail  would  seriously  affect  the  centre  of 
gravity,  the  caudal  vertebrae  are  reduced  in  number, 
and  unite  to  form  the  ploughshare  bone,  with  which 
the  so-called  rectrices,  or  steering  feathers,  are  con- 
nected (Fig.  135;  co).  An  exception  to  the  rule  that 
the  central  portion  of  the  vertebrae  is  that  which  is 
most  completely  ossified  is  afforded  by  the  caudal  ver- 
tebrae of  many  lizards.  In  these  there  is  a  central 
unossified  region,  where,  of  course,  the  tail  is  much 
weaker  than  at  other  points.  If  a  lizard  be  seized  by 
the  tail  it  will  ordinarily  escape,  thanks  to  the  fracture 
of  one  of  these  centra,  while  the  part  of  the  tail  left 
with  the  lizard  will  grow  again.  Here  we  have,  no 
doubt,  an  example  of  a  variation  which  has  been 
seized  upon,  on  the  principle  of  natural  selection,  and 
has  afforded  these  long-tailed  but  inoffensive  forms 
with  a  satisfactory,  though  undignified,  method  of 
protection  and  escape. 

In  fishes  the  vertebral  column  can  only  be 
divided  into  that  which  belongs  to  the  trunk  and  that 
which  belongs  to  the  tail.  In  some  of  the  more  gene- 
ralised the  notochord  extends  in  a  straight  line  to  the 
hinder  end  of  the  body,  dividing  the  tail-fin  into  two 
equal  halves.  In  most  Elasmobranchs  this  notochord 
is  bent  upwards,  and  the  lower  half  of  the  fin  is  much 
larger  than  the  upper.  In  the  Teleostei  the 
notochord  likewise  becomes  so  bent  up,  but  the  rays 
which  support  the  fin  become  so  arranged  as  to  give 
to  the  tail  fin,  when  seen  from  the  surface,  the  appear- 
ance of  being  composed  of  equal  upper  and  lower 
halves,  though,  as  a  matter  of  fact,  all  the  fin  rays  are 


Chap.  IX.] 


SKELETON  OF  FISHES. 


323 


inferior  to  the  notochord,   or  the  modified  urostyle 
which  has  taken  its  place. 

The  unpaired  dorsal  fins  of  Fishes  are  connected 
with  the  vertebral  column  by  means  of  spines,  which 
are  placed  be- 
tween the  neu- 
ral spines  of  the 
vertebrae,  and 
are  connected 
with  the  dermal 
spines  or  fin 
rays  which  sup- 
port and  make 
up  the  greater 
part  of  the  fins 
themselves. 

In  many 
fishes  there  is 
very  constantly 
developed  a 
h?emal  aswell  as 
a  neural  arch  of 
bone  in  connec- 
tion with  the  cen- 
trum, and  these, 
like  the  superior, 
are  produced 
into  a  spine  ; 
in  the  hinder  re- 
gion of  the  body 
interspinous 
bones  are  de- 
veloped between 
these  so-called  hsemaphyphyses,  and  they  serve  to 
carry  the  fin  rays  of  the  anal  fin. 

Articulated  to  the  atlas  is  the  skull,  which,  in  the 
first  place,  is  the  box  or  covering  for  the  brain ;   this 


Fig.  135.— Skeleton  of  Eagle  (reduced). 

c'  Coracoid ;  ca,  carpus  ;  dd',  phalanges  of  the  chief  digit  of  the  wing  ;  d",  pha- 
langes of  smaller  digit;  d'",  pollex;  dr,  dorsal  ribs;  /,  femur ;  JL,  fibula; 
fu,  f urcula ;  h,  humerus;  m,  metatarsus;  ma,  mandible;  me,  metacarpus; 
co,  ploughshare  bone  ;p,  pelvis  ;  pa,  phalanges  of  foot ;  pi,  patella ;  r,  radius; 
sr,  sternal  ribs;  s,  scapula;  st,  sternum;  ti,  tibia,  tm,  Tarsomctatarsus;  u, 
ulna ;  up,  uncinate  processes  of  ribs.  (After  Milne-Edwards.) 


Chap,  i  x.  ]  SKULL.  325 

is  the  true  cranium.  With  this  there  enters  into 
more  or  less  complete  union  the  cartilages  or  bones 
that  form  the  framework  for  the  mouth,  and  give  rise, 
in  higher  forms,  to  the  face. 

The  noto  chord  extends  forwards  below  the  two 
hinder  of  the  three  primitive  brain  vesicles,  and,  on 
either  side,  there  appear  masses  of  cartilage,  homolo- 
gous with  those  that  form  the  arches  of  the  vertebral 
column.  These  parachordals  unite  with  the  noto- 
chord  to  form  a  continuous  toasilar  plate,  which 
serves  as  a  floor  for  the  two  hind  brain  vesicles ;  this 
grows  up  on  either  side,  and  unites  above  to  form  a 
ring  of  cartilage  which  embraces  the  hindermost  part 
of  the  brain.  Posteriorly  each  half  gives  rise  to  a 
cartilaginous  condyle,  which  articulates  with  the 
atlas.  This  hindermost  portion  of  the  cranium  may 
be  distinguished  as  the  occipital  region.  In  front 
of  the  parachordals  there  appear  two  bars,  which  unite 
behind,  where  they  embrace  the  anterior  end  of  the 
notochord,  and  in  front  also,  leaving  a  space  in  the 
middle.  These  are  the  trabeciilae,  and  they  form 
the  floor  for  the  first  brain  vesicle,  or  fore-brain.  As 
the  plates  unite  to  form  a  solid  floor  they  grow  up  at 
the  sides,  but  never  form  more  than  an  imperfect  roof 
in  this  region,  which,  therefore,  is  not  cartilaginous, 
but  membranous.  This  portion  of  the  cranium  may 
be  spoken  of  as  the  sphenoidal  region.  As  the 
cranium  invests  the  brain,  holes,  or  notches  that  will 
be  converted  into  holes,  have  to  be  left  for  the  passage 
of  the  cerebral  nerves  (Fig.  136  ;  5,  9). 

In  addition  to  these,  the  whole  architecture  of  the 
skull  is  profoundly  affected  by  another  set  of  elements, 
which  enter  into  more  or  less  close  contact  with  it. 
These  are  the  capsules  of  the  three  higher  senses,  smell, 
sight,  and  hearing.  At  the  anterior  end  of  the  sphe- 
noidal region  the  olfactory  cartilaginous  capsule 
becomes  connected  with  the  cranium,  the  anterior  wall 


326  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


(ethmoid)  of  which  is  perforated  for  the  passage  of 
the  olfactory  nerves  from  the  brain.  In  the  occipital 
region,  on  either  side,  the  capsule  for  the  ear  (periotic 
capsule)  early  becomes  connected  with  the  cranial 
walls,  and  in  the  higher  Yertebrata  (see  Fig.  136)  the 
periotic  cartilages  are,  at  a  very  early  stage  in  deve- 
lopment, con- 
tinuous with 
the  basilar 
plate.  The 
optic  cartilage 
(sclerotic) 
lies  in  the 
sphenoidal  re- 


CV 


gion, 


but     it 


never  enters 
into  direct 
union  with 
the  cranium, 
though  the 
form  of  this 
part  of  the 
skull  is 
greatly  affec- 
ted by  the 
size  of  the  eye- 
ball. 

In  addi- 
tion to  the  (1) 
cranial  and 
(2)  sensory 
cartilages 

which  take  so  large  a  part  in  the  formation  of 
the  skull,  there  is  in  all  Gnathostomata  yet  a  third 
element,  which  may  be  distinguished  as  the  touccal. 
In  the  branchial  and  visceral  clefts,  which  appear 
just  behind  the  brain,  cartilaginous  bars  are  developed 


Fig.  136.— Cartfagiuous  Cranium  of  a  Chick  of 
the  fourth  Day  of  Incubation,  showing  the 
investing  Mass  (iv),  and  the  Trabeculee  (tr), 
with  their  Central  space  (pts). 

cv,  Cerebral  vesicle  (sliced  off) ;  e,  eye ;  Ig,  anterior  end 
of  investing  mass  formed  from  the  parachordals  ;  5, 
notch  for  the  passage  of  the  fifth  nerve ;  q,  quad- 
rata ;  cl,  cochlea :  she,  semicircular  canal  of  ear ; 
9,  foramen  of  exit  of  the  ninth  nerve ;  we,  noto- 
chord.  (After  Parker.) 


Chap.  IX.]       BUCCAL    ELEMENTS   OF   SKULL.  327 

for  their  support.  Throughout  the  series,  whether 
gills  are  present  or  not,  the  first  two  take  on,  in  addi- 
tion or  solely,  quite  another  than  a  branchial  function. 
These  (Fig.  137  ;  Mrc,  ny)  send  off  from  their 
upper  end  a  process,  which  is  directed  forwards  ;  the 
anterior  arch  becomes  segmented  into  an  upper  and  a 
lower  piece,  both  of  which,  growing  forwards,  form 
the  rudiments  of  the  upper  and  lower  jaws.  The 
former  (pi,  Pt)  may  be  called  the  pterygo-quadrate 
bar,  the  lat- 
ter the  Mec- 
kelian  car- 
tilage. The 
chief  means 
of  connection 
bet  ween  these 
bars  and  the 
cranium  is  , 

not  the  me-         £/        —TOT—     v         ,          ;       R    j 
tapterygo-  Tf   Mn     ***/   J 

idal     region,       Fig  i37._Head  of  Embryo  Dogfish  (11  lines  long) . 

Or  hinder  and  T?%  Trabecula ;  Pi,   Pt,  pterygo-quadrate ;    m,  Pt,  rneta- 

e>  pterygoid ;  M?t,  inandibular  cartilaee  ;  ny,  hyoid  arch ; 

Upper  part  OI  sr    l,  first    branchial    arch,    with  four   succeeding 

,1        />                 -i  arches;  sp,  mandibulobyoid  cleft;  cl,  hyo-lmtnchial 

the  first  arch,  cleft;  Ci,c2,c3,  cere  bral  vesicles.   (After  Parker.) 

but  the  upper 

part  of  the  second  arch  (H?/),  which  forms  the  Ityo- 

mandibular. 

In  such  a  skull,  then,  as  that  of  the  dogfish  (which 
has  formed  the  basis  for  this  account),  the  attachment 
of  the  jaws  to  the  skull  is  hyostylic  (Huxley) ;  in  a 
large  number  of  fishes  this  hyostylic  arrangement 
obtains ;  in  a  few  (Notidanus),  however,  the  meta- 
pterygoid  does  enter  into  contact  with  the  cranium, 
and  the  jaw  is  then  supported  by  elements  of  both  the 
inandibular  and  hyoid  arches,  or  is  ampliistylic.  On 
the  other  hand,  in  Chimsera,  the  Dipnoi,  and  all  the 
pentadactyle  Vertebrata,  the  hyoid  takes  no  share  in 


328  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

attaching  the  jaw  to  the  skull,  that  attachment  being 
effected  solely  by  mandibular  elements,  and  being, 
therefore,  autostylic. 

In  the  branchiate  Yertebrata  the  number  of 
branchial  arches  corresponds  with  that  of  the 
branchiae,  and  the  separate  bars  become  segmented  ; 
all  the  visceral  bars  save  the  mandibular  have  a 
distinct  median  basal  piece,  which  is  known  as 
the  foasiforaiicliial ;  this  passes  on  either  side 
into  the  liypobranchial,  which  is  succeeded 
by  the  ceratotoranchial,  epibrancliial,  and 
pharyiigobranchial.  When  gills  cease  to  be  de- 
veloped these  bars  undergo,  as  may  be  supposed,  a 
certain  amount  of  atrophy,  but,  in  all,  the  first 
branchial  arch  is  retained,  while  in  tortoises  and 
lizards  two  arches  may  be  detected.  These  basal 
portions  always  fuse  with  those  of  the  hyoid  arch,  and 
the  coalesced  pieces  make  up  the  so-called  body  of  the 
hyoid,  which  forms  a  support  for  the  tongue ;  the 
parts  of  the  true  hyoid  arch  form  the  interior,  and 
those  of  the  first  branchial  the  posterior  or  lesser 
cornua  of  the  hyoid  of  man. 

"We  have  hitherto  regarded  the  skull  as  compounded 
of  neural,  sensory,  and  visceral  portions,  all  of  which 
are  formed  by  cartilage ;  we  have  now  to  look  at  the 
same  structure  from  another  point  of  view.  It  has 
already  been  pointed  out,  that  while  the  cartilage  in 
the  occipital  region  of  the  skull  forms  a  complete  ring 
in  its  hinder  portion,  the  sphenoidai  region  is  roofed 
in  by  membrane  ;  this  membranous  roof  is  retained 
throughout  life  by  Myxine  (the  hag).  In  other 
Cyclostomes  and  in  Elasmobranchs  the  roof  becomes 
more  or  less  completely  cartilaginous,  and  this  carti- 
lage, which  never  becomes  ossified,  though  its  outer 
layers  may  be  calcified,  is  covered  in  by  membrane. 

In  the  more  shark-like  Ganoidei,  the  membrane, 
though  not  the  cartilage,  undergoes  ossification,  and  a 


Chap.  IX.J 


SKULL  OF  PISHES. 


329 


number  of  investing  membrane  bones  appear  on 
the  roof  of  the  skull ;  in  the  Holosteous  Ganoids  ossifi- 
cation commences  in 
the  occipital  region  of 
the  cartilaginous  cra- 
nium, while  there  are 
also  membrane  bones. 
From  this  point  for- 
ward we  have  to  dis- 
tinguish between  bones 
that  are  preformed  in 
cartilage  (cartilage 
bones),  and  those 
that  are  preformed  in 
membrane  (mem- 
brane bones). 

At  first,  that  is,  in 
the  lower  Vertebrata, 
the  membrane  bones 
are  numerous,  and 
their  relations  are  not 
so  constant  and  exact 
as  they  are  in  the 
higher  forms.  When 
they  become  so  we  are 
able  to  recognise  that 
the  roof  is  formed  by 
two  pairs  of  more  or 
less  large  bones,  the 
parietals  abutting 
on  the  occipital  re- 
gion, and  thefrontals 
in  front  of  the  parie- 
tals. The  base  of  the 

skull  is  in  the  Ichthyopsida  ossified  in  the  occipital 
region  only,  and  the  sphenoidal  portion  is  under- 
laid by  a  membrane  bone,  the  parasphenoid 


Fig.  138.— Head  of  Sturgeon,  showing  the 
Membrane  Bones,  aud  the  Cartilaginous 
Cranium,  which  is  shaded  dark.  (After 
Gegenbaur.) 


330    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


(Fig.  139  ;  par).  The  upper  half  of  the  mandibular 
arch  becomes  invested  by  membrane  bones  only,  the 
jugal  or  quadratojugal,  or  both,  which  trend  a 
little  inwards  as  they  pass  forwards  (Fig.  139  ;  q). 
In  front  of  these  are-  two  bones  which,  typically,  carry 
teeth,  the  maxillae  and  the  premaxillse. 

Internal  to  this  row  of  membrane  bones  is  another, 
of  which  the  most  anterior,  the  vomer  (v),  is  formed 
from  membrane  that  did  not  overlay  cartilage,  just 
like  the  maxillae  and  the  premaxillse,  while  the 
others,  the  palatines  and  the  pterygoids  (pt),  are 

formed  from  mem- 
brane which  gene- 
rally invested  carti- 
laginous bars.  Mec- 
kel's  cartilage  is 
likewise  invested  in 
bones  of  membra- 
nous origin,  the  most 
important  and  con- 
stant of  which  is 
the  tooth  -  bearing 
dentary*  At  the 
anterior  end  of  the 
skull,  above  the 
olfactory  capsule, 
there  appear  the 
paired  nasals,  with 
which  a  lachrymal 
is  related  in  the 
higher  forms. 

In  Fishes  a  series  of  membrane  bones  may  become 
developed  in  connection  with  the  branchial  skeleton, 
and  form  the  support  for  the  opercular  flap  of  the 
gills;  such  are  the  opercnlnm,  siiboperculum, 
and  interoperculum.  The  most  anterior  of  the 
opercular  bones  is  possibly  the  homologue  of  the 


Fig.  139.— Skull  of  Frog,  from  below; 
the  Lower  Jaw  lias  been  removed. 

e,o,  Exoccipital ;  po,  prootic ;  par,  parasphe- 
noid  ;  et  sphenethmoid  ;  v,  vomer;  pm, 
premaxilla-  mx,  maxilla;  q,  quadrato 
Jugal ;  pt,  pterygoid  ;  SMS,  suspensorium  ; 
palatine ;  1,  optic  foramen ;  2,  foramen 
of  fifth  nerve  ;  3,  foramen  for  ninth  and 
tenth  nerves.  (After  Parker.) 


chap,  ix.j  SKULL  OF  AMNIOTA.  331 

membranous  bone  at  the  side  of  the  skull,  which  is 
known  as  the  squamosal  in  the  abranchiate  Verte- 
brata. 

While  in  the  Amphibia  the  posterior  (occipital) 
and  anterior  (ethmoid)  portions  of  the  base  and  sides 
of  the  cartilaginous  cranium  undergo  ossification,  it  is 
riot  till  we  reach  the  Amniota  that  we  find  the 
central  and  lateral  cartilaginous  parts  of  the  sphenoidal 
region  becoming  bony  ;  when  they  do  so  we  recognise 
a  basisphenoid,  with  an  alisphenoid  on  either 
side,  and  a  presphenoid  with  corresponding  later- 
ally placed  orbitosphenoids.  Now,  too,  we  can 
distinctly  see  an  ossified  basioccipital,  two  ex- 
occipitals, and  a  median  supraoccipital,  all  of 
cartilaginous  origin,  and  surrounding  the  foramen 
magnum.  In  the  Saurcpsida  the  exoccipitals  unite 
with  the  basioccipital  to  form  a  single  median 
occipital  condyle ;  in  the  Mammalia  the  exoccipitals,  as 
in  the  dog,  alone  form  the  condyles,  or  some  share  is 
taken  by  the  basioccipital,  but  in  either  case  the  skull 
is  articulated  to  the  vertebral  column  by  two  con- 
dyles ;  it  is  for  this  reason  that  some  writers  speak  of 
the  Sauropsida  as  Monocondyla,  and  of  the  Mammalia 
as  Amphicondyla. 

At  the  anterior  end,  the  cartilaginous  plates  which 
subdivide  the  nasal  cavity  may  undergo  more  or  less 
ossification,  and  give  rise  to  the  "  spongy  bones " 
of  the  nose  ;  they  enter  into  connection  with  tho 
ethmoid  in  the  middle  line,  and  may  become  united 
with  the  nasals  (naso  -  turbinals)  or  maxillae 
(maxillo-turbinals)  at  the  sides.  (See  Fig.  189, 
page  442.) 

In  the  walls  of  the  cartilaginous  ear- capsule  there 
appear  centres  of  ossification,  which  are  ordinarily 
three  in  number ;  of  these  the  most  constant  is  the 
prootic,  which  alone  is  found  in  the  Amphibia,  though 
in  some  fishes  there  are  also  epiotic  and  opisthotic 


332  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

ossifications.     In  Birds  these  fuse  with  one  another 
and   with   the   supra-  and  ex-occipital   bones   to   an 


Fig.  140. — Diagram  of  the  Cranial  Bones  of  a  Mammal,  showing  the 
Foramina  of  exit  of  the  several  Cerebral  Nerves.* 

1,  Olfactory  nerve  ;  2,  optic ;  of,  optic  foramen  ;  da,  foramen  lacerum  anterium,  for 
the  passage  of  the  third,  fourth,  sixth,  and  first  branch  of  fifth  ner\re  (3,  4, 
6,  5<x; ;  fr,  foramen  rotundum,  for  the  second  branch  of  the  fifth  (5)3) ;  fo, 
foramen  ovale,  for  the  third  branch  of  the  fifth  (ay") ;  am/,  stylomastoid 


foramen  for  the  seventh  nerve  (7) ;  flp,  foramen  lacerum  posterius,  for  the 
ninth,  tenth,  and  eleventh  nerves  ;  MES,  mesethmoid  ;  cf,  condylar  foramen 
for  the  twelfth  nerve ;  CP,  cribriform  plate :  PS,  presphenold ;  BS,  basi- 


sphenoid;  BO,  basioccipital ;    os,  orbitosphenoid ;  AS,  aiisphenoid ;  E'O,  ex- 
occipital  ;  PEE,  periotic ;  F«,  frontals  ;  PA,  parietals ;  so,  supraoccipital. 

extent  of  completeness  which  is  greater  than  ordinarily 
obtains  among  Mammals. 

In  addition  to  the  important  relations  which  the 
cranium  bears  to  the  sense  capsules,  it  has  others  even 

*  This  diagram  was,  many  years  ago,  shown  to  the  author  by 
Prof.  Flower,  F.R.S.,  and  it  is  here,  for  the  first  time,  published 
by  his  kind  permission. 


Chap,  ix.}  FORAMINA  OF  SKULL.  333 

more  important  to  the  brain,  which  it  contains  and 
protects  ;  of  these  the  most  important  is  its  relation  to 
the  cerebral  nerves  and  outgrowths  that  pass  out 
from  it.  The  distribution  and  arrangement  of  these 
nerves  form,  in  disputed  cases,  one  of  the  best  criteria 
of  the  homologies  of  the  different  parts  of  the  cranium. 
Seen  in  its  most  elaborated  condition,  as  it  is  found  in 
Mammals,  the  cranial  bones  and  cerebral  nerves  have 
the  following  relations.  The  so-called  olfactory  nerve 
(Fig.  140  ;  1)  perforates  the  cribriform  plate  of  the 
ethmoid ;  the  optic  nerve  passes  through  the  optic 
foramen  in  the  orbitosphenoid  bone  (os) ;  the  third, 
fourth,  and  sixth  nerves,  that  go  to  the  muscles  of  the 
eye,-  pass  with  the  most  superior  division  of  the  fifth 
through  the  irregular  or  jagged  space  (/la)  that  lies 
between  the  orbitosphenoid  and  the  alisphenoid.  The 
two  other  branches  of  the  fifth  (or  trigeminal)  pass 
through  the  round  (fr)  and  oval  (fo)  foramina  in 
the  alisphenoid,  while  the  seventh  has  a  passage  at  the 
outer  side  of  the  periotic  (PER),  while  between  the 
periotic,  basioccipital,  and  exoccipital  there  is  a  pos- 
terior foramen  lacerum  (flp)  for  the  glossopharyngeal, 
vagus,  and  hypoglossal  (9,  10,  11)  nerves;  lastly,  the 
twelfth  nerve  passes  through  the  conclylar  foramen  (cf), 
while,  as  we  have  already  learnt,  there  is  a  great 
foramen  at  the  hinder  end  of  the  cranium  which 
serves  as  the  means  by  which  the  medulla  oblongata 
is  allowed  to  be  continuous  with  the  spinal  cord. 

An  examination  of  the  interior  of  the  cranium 
similarly  reveals  the  close  connection  that  obtains  be- 
tween the  containing  case  and  the  contained  brain ; 
and,  indeed,  our  knowledge  of  the  characters  of  the 
brains  of  extinct  forms  is  absolutely  dependent  on  casts 
of  the  internal  configuration  of  such  skulls  as  have 
been  preserved  to  us  in  the  form  of  fossils. 

Where,  as  in  the  lower  Mammals,  the  cerebral 
hemispheres  are  of  no  great  size,  and  do  not  overlap 


334    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  hinder  cerebellum,  the  upper  surface  of  the  cranium 
is  straight  and  flattened  \  as  we  ascend  the  scale,  how- 
ever, we  find  that  the  cerebral  hemispheres  growing 
backwards  come  to  overlap  more  or  less  the  cerebellar 
region ;  concurrently  with  this  the  upper  surface  of 
the  brain  becomes  more  or  less  arched,  and  the  cranial 
walls  take  on  a  corresponding  form  ;  the  most  familiar 
example  of  this  is,  of  course,  the  brain  of  man,  but  it 
is  to  be  carefully  noted  that  the  skulls  of  the  old-world 
baboons,  and  of  some  of  the  lower  and  smaller  new- 
world  monkeys,  have  the  supraoccipital  region  thrown 
farther  back  and  down  than  it  is  in  man  himself.  As 
a  result  of  this  alteration  in  the  position  of  the  parts 
of  the  brain  case  we  find  that  the  foramen  magnum 
looks  downwards  instead  of  backwards  ;  as  a  secondary 
result  we  find  that  the  skull  of  man  balances  more  or 
less  completely  on  the  occipital  condyles,  and,  this 
being  so,  there  is  not  the  same  need  for  the  development 
of  muscles  and  ligaments  to  support  and  hold  up  the 
back  of  the  head  as  there  is  in  the  dog  or  the  horse  ; 
from  this  mechanical  arrangement  we  get,  further,  a 
marked  diminution  in  the  extent  of  the  bony  ridges 
on  the  occiput  to  which  these  muscles  are  attached. 

When  a  longitudinal  section  is  made  through  the 
skull  of  a  Mammal  and  the  form  of  the  internal  cavity 
is  revealed,  it  is  seen  that  the  bony  ear-case  projects 
into  the  hinder  part  of  the  cavity,  and  that  the  wall  of 
the  anterior  boundary  is  perforated  by  the  small  holes 
which  give  passage  to  the  fibres  of  the  olfactory  nerve 
(cribriform  plate  of  the  ethmoid) ;  in  the  whales,  where 
the  olfactory  sense  is  in  a  rudimentary  condition,  the 
holes  in  this  plate  are  few  and  small ;  the  region  of  the 
skull  in  which  the  olfactory  lobes  of  the  brain  are  con- 
tained is  known  as  the  olfactory  fossa,  and  this  is 
smaller  or  larger  according  to  the  size  of  the  olfactory 
lobes  themselves  (see  page  425)  ;  this  cavity  is  bounded 
by  the  cribriform  plate  in  front  and  below,  and  at  the 


Chap,  ix.]        MOUTH  OF  CYCLOSTOMATA.  335 

sides,  and  has  behind  it  a  ridge  of  bone  on  the  orbito- 
sphenoid,  and  frontal  bones  by  which  it  is  separated 
from  the  cerebral  fossa,  which,  in  all  Mammals, 
occupies  a  larger  part  of  the  cranial  cavity  ;  this  fossa 
is  more  or  less  feebly  divided  into  two  by  a  ridge  of 
bone  which  corresponds  to  the  sylvian  fissure  of  the 
brain  ;  in  the  more  anterior  division  there  lies  the 
frontal  lobe  (see  page  426) ;  behind  this  comes  the  cere- 
bellar  fossa,  marked  off  anteriorly  by  the  tentoriuni. 
The  flocculus  of  the  cerebellum  lies  in  a  special  depres- 
sion on  the  inner  face  of  the  periotic,  and  the  hypo- 
physis cerebri  on  a  pit  (sella  turcica)  on  the 
superior  face  of  the  basisphenoid,  which,  as  we  have 
already  learnt,  forms  a  portion  of  the  floor  of  the  brain 
cavity. 

Such,  then,  being  the  general  disposition  of  the 
parts  of  the  skull  in  a  well-developed  Vertebrate,  we 
have  now  to  investigate  the  arrangements  which  obtain 
in  various  forms  in  relation  to  their  habits  of  life  and 
their  zoological  affinities. 

In  the  Round-mouths,  where  no  branchial  rods 
are  modified  to  form  jaws,  the  sides  of  the  buccal  orifice 
are  supported  by  cartilaginous  pieces,  the  so-called 
labial  cartilages  ;  the  mouth  is  surrounded  by  a  circu- 
lar lip,  in  the  posterior  region  of  which  is  placed  the 
annular  cartilage ;  into  the  cavity  of  the  mouth 
there  project  a  number  of  horny  denticles  ;  on  the 
floor  is  the  lingual  cartilage,  and  in  front  of  this  is  the 
median  ventral  cartilage,  which,  possibly,  represents 
the  basal  median  portion  of  the  mandibular  arch,  which 
in  other  Vertebrates  is  only  to  be  detected  in  early 
stages.  It  is  very  instructive  to  observe  that  there  is 
a  close  resemblance  between  the  mouth  parts  of  a 
lamprey  and  those  of  a  tadpole  during  the  period  when 
the  latter  has  a  suctorial  mouth. 

The  labial  cartilages  of  the  Cyclostomata  appear  to 
be  better  retained  by  Elasmobranchs  than  by  other 


336  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Vertebrates,  in  which  only  those  persist  that  take  part 
in  the  formation  of  the  nasal  cavities. 

In  the  voracious  sharks  the  mouth  is  of  large  size, 
but  the  character  of  their  food  is  related  rather  to  the 
size  of  their  teeth  than  to  the  extent  of  their  mouth ; 
when  the  mouth  is  not  terminal  but  ventral  in  position, 
a  shark  has  to  turn  on  its  side  to  seize  its  prey. 
In  some  Rays,  such  as  Pristis  (the  saw-fish)  the 
snout  is  produced  into  a  long  flattened  weapon  of 
attack,  which  is  armed  at  the  sides  with  spinous 
processes,  and  serves  as  an  organ  by  means  of  which 
the  body  of  its  prey  may  be  torn  open.  In  the 
Teleostean  sword-fishes,  where  maxillae  and  premaxill<e 
are  present,  these  bones  are  produced  into  a  long 
stabbing-organ,  not  only  strong  enough  to  pierce  the 
bodies  of  whales,  but  even  the  planks  of  wooden 
ships.  In  many  Ganoidei,  such  as  the  sturgeon,  the 
snout  is  of  considerable  length ;  the  function  of  this 
organ  is  not  completely  understood,  and  the  most 
plausible  hypothesis  is  that  of  von  Martens,  who 
ascribes  to  the  long  snout  of  Polyodon  the  function 
of  a  tactile  organ,  the  necessity  for  which  is  to  be 
explained  by  the  turbidity  of  the  rivers  in  which  it  lives. 

In  the  Teleostei  the  mouth  may  be  very  large,  as 
in  the  Angler  (Lophius),  or  exceedingly  small,  as  in 
Chsetodon  and  Diodon  ;  in  Chelmo,  a  form  allied  to 
Chsetodon,  the  mouth  is  prolonged  into  a  snout  which 
possibly  serves  as  an  apparatus  for  drawing  from  holes 
or  crevices  the  small  animals  on  which  it  feeds 
(Giinther).  Where  the  teeth  are  of  great  size,  and 
adapted,  say,  for  crushing  shells,  as  in  the  sea-cat,  the 
jaws  which  carry  them  are  of  corresponding  strength;  in 
the  wrasses,  with  somewhat  similar  habits,  the  upper 
pharyngeal  bones  are  articulated  with  the  basi-occipi- 
tals,  and  no  doubt  afford  a  firmer  fulcrum  for  the  jaws. 

In  bony  fishes  the  eye-ball  is  sometimes  provided 
with  separate  bony  pieces  for  its  protection. 


Chap.  IX.} 


SKULL  OF  OPHIDIA. 


337 


Pmx 


PL 


In  the  Amphibia  the  mouth  is  wide,  and  in 
some  Anura  the  symphysis  of  the  mandibles  is  strong, 
for  it  has  to  serve  as  the 
point  of  attachment  of  the 
tongue  (see  page  154) ;  in 
some  TJrodela,  as  in  some 
Reptiles  and  Birds,  the 
sclerotic  cartilage  undergoes 
ossification,  but  presents  only 
an  advance  on  what  is  seen 
in  the  Teleostei. 

Owing  to  our  better 
knowledge  of  the  habits  of 
the  higher  Vertebrates,  we 
are  more  easily  able  to 
associate  the  arrangements 
of  parts  of  their  skulls  with 
the  habits  of  their  possessors.  * 
Nowhere  do  we  find  a  better 
series  of  mechanical  arrange- 
ments than  among  the 
Optiidia;  in  the  python 
and  those  that  swallow, 
without  poisoning,  their 
prey,  the  skull  is  wide  be- 
hind, and  the  quadrates  are 
at  a  considerable  distance 
from  one  another  (Fig.  141 ; 
QM).  In  other  words,  the 
lower  jaw  is  wide  at  its 
point  of  attachment,  and, 

as  we  have  learnt  already  (page  96),  the  quadrate 
is  movable  upon  the  squamosal  (sq),  so  that  the 
width  of  the  mouth  behind  can  be  very  considerably 
increased. 

Anteriorly,    there    is    a    corresponding    arrange- 
ment,  inasmuch    as   the   lower  jaws  are   not  firmly 
w— 16 


Fig.  141.— Lower  Surface  of 
Skull  of  Python. 

praz,  Premaxillae  ;  MX,  maxillse ; 
vo,  vomer;  IT,  transverse 
bone  ;  pi,  palatine  ;  Ft,  ptery- 
goid;  Q«,  quadrate;  sq, sciua- 
niosal;  BS,  basisphenoid; 
BO,  basioccipital.  (After 
Huxley.) 


33  8  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

united  by  a  symphysial  suture,  but  are  only  con- 
nected by  an  elastic  ligament.  Thanks  to  this  dis- 
position of  the  separate  parts  in  front  and  behind, 
the  cavity  of  the  mouth  is  capable  of  very  considerable 
enlargement. 

In   the   venomous   snakes   the   arrangements  are 


M/t 


Fig.  142.— Skull  of  Crotalus. 

Pmp,   Premaxilla;  Ma;,   maxilla;    La,  lachrymal;  vl,  palatine;   Ft,  pterygoid ; 
QU,  quadrate;  MM,  lower  jaw.    (After  Huxley.) 

even  more  elaborate ;  the  maxilla,  instead  of  being 
elongated  backwards,  is  shortened  antero-posteriorly, 
so  that  its  longer  axis  is  directed  downwards ;  it 
carries  one  long  tooth,  the  "poison  fang,"  and  is 
excavated  on  its  upper  outer  surface,  where  there  lies 
the  duct  of  the  poison  gland  ;  the  maxilla  moves  freely 
on  the  lachrymal,  which  again  can  move  en  the 
frontal.  The  palato-pterygoid  bar  (Fig.  142  ;  P£,  pt)  is 
jointed,  and  the  pterygoid  is  further  connected  with 


chap,  ix.]  SKULL  OF  OPHIDIA.  339 

the  maxilla  by  the  transverse  bone  (tr).  The  long 
quadrate  is  movable  in  a  plane  parallel  to  the  Icng 
axis  of  the  skull,  and  when  the  mouth  is  shut  is 
directed  backwards,  so  that  the  palato-pterygoid  bar 
forms  a  straight  line,  while  the  palatine  and  transverse 
bones  pull  the  maxilla  backwards  and  upwards,  and 
so  cause  the  poison  fang  to  lie  along  the  floor  of  the 
skull.  When  the  mouth  opens,  the  point  of  attach- 
ment of  the  lower  jaw  is  necessarily  drawn  upwards  ; 
this  action  forces  the  lower  end  of  the  quadrate 
forwards  ;  the  quadrate  acts  on  the  pterygoid,  and  the 
pterygoid  on  the  palatine  and  transverse  bones,  which, 
driving  forwards  what  is  before  them,  force  the 
maxilla  outwards  and  downwards ;  the  poison  fang  is 
thus  caused  to  become  vertical,  or  in  a  position  to 
inject  the  poison,  which  is  simultaneously  forced  out 
of  its  gland  through  the  channel  in  the  tooth,  and  so 
into  the  body  of  the  victim. 

In  the  poisonous  Mexican  lizard  Heloderma,  the 
poison  glands  are  modifications  of  sublingual  glands, 
and  the  lower  jaw  is  traversed  by  the  channels 
through  which  their  ducts  pass  into  the  floor  of  t'he 
mouth.  In  Hatteria,  the  teeth  are  lost  after  a 
certain  time,  and  their  place  is  taken  by  the  bones 
that  bear  them,  the  margins  of  which  are  sharp  and 
of  exceedingly  dense  structure,  so  that  here  the  bones 
take  the  place  of  the  teeth.  (See  page  146.) 

While  in  the  Ophidia  the  two  halves  of  the  lower 
jaw  are  connected  by  ligament,  and  in  most  Lizards 
and  all  Crocodiles  by  sutures,  they  become  completely 
fused  with  one  another  in  the  Chelonia;  the  same 
phenomenon  is  to  be  observed  in  Birds,  and  in  both 
cases  it  is,  no  doubt,  to  be  correlated  with  the  loss  of 
teeth  and  the  acquirement  of  a  horny  investment  or 
beak.  In  the  turtles  the  fossa  on  either  side  of  the 
skull,  bounded  externally  by  the  jugal  and  quadrato- 
jugal,  is  roofed  over  by  an  outgrowth  of  the  parietal ; 


340  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

this  remarkable  arrangement  is,  curiously  enough, 
found  also  in  one  species  of  the  amphibian  Pelobates, 
and  in  the  African  rodent  Lophiomys. 

In  the  voracious  Crocodiles  the  teeth  are  con- 
fined to  the  maxillse,  premaxillsR,  and  lower  jaw,  as  in 
Mammals,  and  these  bones  are,  further,  socketed  for 
the  reception  of  the  strong  teeth.  In  connection  with 
their  mode  of  life,  which  requires  that  they  should, 
while  breathing  air,  hide  as  much  of  their  body  as 
possible  under  the  water,  the  anterior  iiares  are 
placed  at  the  end  of  their  long  snout ;  the  eye,  for  the 
same  reason,  lies  high  up  on  the  head,  and  the  orbital 
cavity  is  seen,  therefore,  from  an  upper  rather  than 
from  a  lateral  view  of  the  skull ;  the  hinder  openings 
of  the  nasal  passages,  which,  in  the  forms  hitherto 
described,  lie  in  the  anterior  region  of  the  roof  of  the 
mouth,  are  in  the  crocodile  placed  very  far  back  ;  this 
is  effected  by  the  development  of  transverse  plates 
formed  by  the  maxillse,  palatines,  and  pterygoids 
which  unite  into  a  long  floor  for  the  nasal  passages. 

In  many  Birds  the  upper  jaw  is  capable  of  a 
certain  amount  of  vertical  movement,  such  as  is 
carried  to  an  extreme  in  the  parrots ;  this  freedom  is 
due  either  to  incomplete  ossification  between  the 
ethmoidal  and  nasal  regions,  or  to  the  formation 
of  true  articulations  between  the  large  premaxillse 
on  the  one  hand,  and  the  f rentals,  jugals,  and  pala- 
tines on  the  other;  these  latter,  as  in  a  number 
of  other  birds,  are  not  immovably  connected  with 
the  basisphenoid,  but  can  slide  backwards  and 
forwards,  while  the  quadrate,  with  which  the  jugal 
is  connected,  is  movable,  as  in  lizards  or  snakes. 
When  the  quadrate  is  pushed  forwards,  as  it  is  when 
the  lower  jaw  is  depressed,  it  pushes  the  jugal  forwards, 
and  that  bone  pushes  the  beak  upwards.  While  the 
maxillse  are  comparatively  small,  the  three-rayed  pre- 
maxillse  are  of  considerable  size,  and  the  processes  by 


Chap.  IX.] 


SKULL  OF  BIRDS. 


which  they  are  connected  with  the  frontal  and  with  the 
palatines  are  very  important  factors  in  the  attachment  * 
of  the  beak  to  the  cranium  ;  especially  is  this  the  case 
in  those  skulls  in  which  ossification  of  the  ethmoidal 
region  is  incomplete. 

In  the  wood-peckers,  where  the  head  is  "  employed 


Fig.  143.-Side  view  of  a  Dissected  Head 
of  a  common  Wood-pecker.  (Half  iiat. 
size.) 

u  i,  Upper  and  lower  jaws  ;  t,  barbed  tip  of 
tongue ;  thh,  thyrobyal  of  right  side,  with 
its  muscle  and  sheath ;  o,  right  orbit ;  n. 
right  nostril;  sgr,  salivary  gland;  m,  m, 
muscles  of  neck ;  o»,  oesophagus  ;  tr,  trachea  ; 
rm,  retractor  muscles  of  tongue.  (.After 
Macgillivray.) 


as  a  powerful  hammer  or  axe,  whose  strokes  can  be 
heard  at  a  considerable  distance  "  (Garrod),  the  bones 
of  the  skull  are  thicker  and  stronger  than  in  most 
birds,  and  the  upper  jaw  has  its  power  of  vertical 
movement  considerably  limited.  The  articulation  of 
the  palatine  with  the  basisphenoid  disappears,  in 
consequence  of  the  feeble  development  of  the  hinder 


342   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

end  of  the  palatines.  In  correlation  with  the  mode  of 
action  of  the  tongue  (Fig.  143  ;  't\  which  is  protruded 
into  the  recesses  from  which  the  bird  extracts  the 
insects  on  which  it  feeds,  the  hyoid  bones  that 
support  it  (thJi)  are  greatly  developed,  the  anterior 
cornua  passing  backwards  and  then  curving  forwards 
over  the  roof  of  the  skull,  in  grooves  of  which  they 
lie.  From  the  ends  of  these  bones  or  their  sheath, 
there  arises  a  muscle  which  is  inserted  into  the  lower 
part  of  the  same  bones ;  the  contraction  of  this  muscle 
straightens  the  curve  of  the  thyrohyals,  but  this  can 
only  be  effected  by  the  protrusion  of  their  free  or 
lower  ends ;  as  the  tongue  is  placed  at  this  point,  and 
is  supported  by  the  bones,  it  is  clear  that  it  is 
simultaneously  protruded.  (See  page  156.) 

In  the  Mammalia  the  quadrate,  of  which  we 
have  till  now  heard  so  much,  is  converted  into  one 
of  the  auditory  ossicles,  the  malleus ;  the  covering 
bones  of  the  mandible  are  reduced  to  the  dentary. 
Man,  so  far  as  is  known,  is  the  only  animal,  in  addition 
to  the  frog  and  the  sturgeon,  in  which  the  meiito- 
meckeliaii  cartilage  is  found  on  either  side  of 
the  mandibular  symphysis.  The  Marsupials  are  re- 
markable for  having  the  angle  of  their  lower  jaw 
developed  into  a  strong  inflection,  which  is  absent 
only  in  Tarsipes,  and,  among  the  Eutheria,  has  only 
been  observed  in  the  insectivorous  Centetes  (the 
Tenrec). 

When  we  compare  a  carnivorous  mammal,  in 
which  the  lower  jaw  moves  up  and  down,  with  a 
ruminant,  in  which  it  moves .  from  side  to  side,  we 
observe  a  difference  in  the  characters  of  the  glenoid 
cavity  by  which  the  mandible  is  articulated  with  the 
cranium  ;  in  the  dog,  for  example,  this  fossa  is  concave 
from  before  backwards,  and  the  projections  in  front 
and  behind  limit  the  movements  of  the  articular  end 
of  the  mandible  to  such  as  can  be  effected  in  a  vertical 


Chap.  IX.] 


SKULL  OF  ELEPHANT. 


343 


direction  ;  and  this  articular  end  or  condyle  is  convex. 
In  the  sheep,  on  the  other  hand,  the  condyle  is  broad 
and  flattened,  and  works  upon  a  flattened  or  slightly 
convex  glenoid  facet. 

When   a   large  surface    of    attachment    for    the 


Fig.    144.— Section  of  the  Skull  of  an  Indian  Elephant,  to  show  the 

Air  Cavities. 
6,  Brain-case  ;  s,  air  sinuses ;  n,  nostril ;  m,  molar ;  t,  tusk. 

necessary  muscles  is  required,  as  in  the  elephant,  the 
skull  is  not  composed  of  thick  bone  throughout,  .but 
a  finer  bony  tissue  with  a  number  of  intervening  air 
cells  is  developed  between  the  "  tables  "  of  the  cranial 
bones ;  by  these  means  a  wider  area  is  obtained 
without  any  proportional  increase  in  the  weight  of 
skull  to  be  supported  (Fig.  144). 


344  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

In  the  dolphins,  where  the  olfactory  sense  is  greatly 
reduced  or  perhaps  altogether  lost,  no  olfactory  fossa 
is  to  be  seen  in  a  longitudinal  section  of  the  cranium  ; 
in  correlation  with  their  habits,  and  the  large  size  of 
the  mouth  cavity,  and  the  length  of  the  premaxillse 
and  maxillae  which  form  its  roof,  the  nasals  are 
reduced  in  size  and  placed  far  up  on  the  surface  of  the 
skull ;  the  anterior  nares  consequently  look  upwards 
instead  of  forwards,  and  are  therefore  the  first  to 
appear  above  the  surface  of  the  water,  when  these 
pulmonate  aquatic  animals  come  up  to  breathe  air. 
Just  as  in  crocodiles,  whose  habits  are  so  far  essentially 
similar,  the  ptery golds  develop  transverse  plates, 
the  formation  of  which  throws  the  posterior  nares 
very  far  back  on  the  roof  of  the  mouth,  and  thereby 
approximates  them  to  the  opening  into  the  trachea. 

It  is  important  to  observe  that  quite  a  different 
set  of  arrangements  obtains  in  the  Sirenia,  which, 
instead  of  being  active  or  marine  forms,  are  sluggish, 
and  live  much  at  the  bottom  of  shallow  rivers ;  here 
the  bones  are  exceedingly  dense,  the  anterior  nares 
look  forwards,  but  are  of  very  large  size,  and  no  special 
plates  of  the  pterygoid  prolong  the  nasal  passages 
backwards.  When  we  compare  the  structural  adap- 
tations of  the  Sirenia  and  the  Cetacea,  it  is  impossible 
to  avoid  seeing,  on  purely  theoretical  grounds,  which 
of  the  two  has  the  advantage  ;  and  this  consideration 
may  be  supported  by  the  abundance  of  the  species 
and  specimens  of  the  latter  as  compared  with  the 
former. 

The  ant-eater  is  another  form  in  which  the 
posterior  nares  are  placed  very  far  back  owing  to  the 
development  of  pterygoidal  plates  to  form  a  floor  for 
the  air  passages ;  in  this  case  an  explanation  is  possibly 
to  be  found  in  the  fact  that  such  a  disposition,  by 
means  of  which  the  nares  and  the  glottis  are  brought 
into  much  closer  proximity,  would  be  an  advantage  in 


chap,  ix.i  HIES.  345 

forms  where  a  powerful  inspiration  might  lead  to  an 
insect  being  carried  from  the  tongue  into  the  trachea. 

In  various  Mammals  the  roof  of  the  skull  is  in  one 
way  or  another  enlarged ;  in  the  sperm-whale,  the 
nasals,  maxillaries,  and  supraoccipital  unite  to  form  a 
large  basin  ;  as  the  contents  of  this  basin  are,  how- 
ever, of  an  oily  nature,  we  may  believe  that  the 
specific  gravity  of  the  bone  is  neutralised  (even  if  no 
advantage  is  gained)  by  that  of  the  spermaceti,  the 
specific  gravity  of  which  is  '843  at  50°  C.  In  the 
Rhinoceros  and  in  the  Uiigulata  the  roof  of  the  skull 
carries  horns.  (See  page  369.)  In  the  porcupines 
many  of  the  upper  cranial  bones  are  expanded,  and 
contain  large  air  sinuses,  as  in  the  frontal  region  of 
the  sloths. 

The  morphological  characters  and  homologies  of 
the  ribs  present  a  problem  that  is  yet  unsolved  ;  the 
evidence  as  to  their  origin  in  the  embryo  being  not  a 
little  conflicting.  In  a  number  of  forms  they  un- 
doubtedly arise  in  the  tissue  of  the  protovertebrae 
(see  page  529)  quite  independently  of  the  vertebrae 
themselves,  their  connection  with  which  is  only 
secondary.  In  the  Cyclostomata,  Holocephala,  and 
some  rays,  there  are  110  ribs,  and  the  supporting 
function  of  these  absent  rods  is  undertaken,  as  in 
Amphioxus,  by  the  fibrous  tissue  which  lies  between 
the  dorsal  and  ventral  muscular  layers. 

Among  the  Amphibia  and  the  Amniota,  the  ribs, 
when  present,  and  typically  developed,  are  connected 
with  the  vertebrae  by  an  upper  and  lower  process  ;  in 
the  Csecilise,  where,  among  amphibians,  ribs  are  best 
developed,  these  two  processes  are  widely  separated  3 
in  the  Salamanders  the  proximal  ends  of  the  anterior 
ribs  are  forked,  and  of  the  hinder  single  ;  in  the 
Anura  the  ribs  are  reduced  or  quite  rudimentary. 

In  the  Amniota  the  ribs  have  a  greater  functional 
importance,  as  is  shown  by  the  length  of  the  more 


346  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

anterior,  and  their  union  in  many  cases  with  the 
sternum  in  the  ventral  median  line.  A  definite 
thorax  may  henceforward  be  recognised,  and  the 
appearance  of  this  cage  associated  with  the  complete 
absence  of  branchial  respiration.  In  front  of  the 
thorax  more  or  less  well  developed  ribs  may  be 
developed,  but  these  never  become  connected  with  the 
sternum ;  in  the  Chelonia  there  are  no  cervical  ribs. 
The  sternal  ribs  are  jointed,  and  in  Reptiles  the 
sternal  portion  remains  cartilaginous ;  where  the 
sternum  is  short  many  of  the  ribs  are  connected  with 
the  cartilaginous  rod  that  lies  behind  it.  In  Birds 
the  connection  between  the  ribs  and  the  sternum  is 
more  complete,  and  the  whole  apparatus  is  firmer  and 
stronger  (see  Fig.  135),  while  in  them,  as  in  crocodiles, 
the  side  walls  of  the  thorax  are  strengthened  by  the 
development  of  backwardly  directed  processes  from 
the  ribs  (uncinate  processes)  (up).  Cervical  ribs 
are  very  rare  among  Mammals,  though  occasionally 
found,  as  in  Bradypus.  The  thoracic  ribs  are  so 
articulated  as  to  be  capable  of  a  considerable  amount 
of  movement,  thanks  to  which  the  thoracic  cavity 
may  be  increased  or  diminished  in  size  ;  they  are 
either  attached  directly  to  the  sternum,  or  connected 
with  it  by  ligament,  or  are  quite  free  at  their  distal 
end.  In  the  Cetacea  some  of  the  hinder  ribs  lose 
even  their  attachment  to  the  vertebrae. 

In  the  Ophidia,  where  the  primitive  condition  of  a 
large  number  of  similar  ribs  is  retained,  there  is  a 
single  articular  head  for  connection  with  the  vertebral 
column  ;  this  is  of  a  kind  to  allow  of  considerable 
freedom  of  movement,  and  the  snake  uses  its  successive 
pairs  of  ribs  as  stilts  by  which  it  may  move  along  the 
ground. 

In  the  Amniota  the  ventral  ends  of  the  ribs 
become  constricted  off,  and  fuse  with  one  another  in 
the  median  line ;  the  cartilaginous  plate  thus  formed 


Chap.  IX.] 


STERNA. 


347 


is  known  as  the  sternum.  With  the  anterior  end 
of  this  sternum  there  ordinarily  comes  into  relation 
the  ventral  part  of  the  anterior  limb-arch  (see  infra)  ; 
and,  farther  back,  we  may  find  the  ventral  attach- 
ments of  the  ribs.  In  the  Chelonia  and  Ophidia 
the  sternum  is  lost,  and  this  is  to  be  correlated  with 
the  great  development  of  the 
exoskeleton  of  the  one,  and 
the  mode  of  life  of  the  others 
of  these  orders  of  Reptiles. 
In  the  Carinate  birds  the  me- 
dian portion  of  the  sternum  is 
produced  into  a  long  and 
strong  keel  (Fig.  145  ;  cs),  to 
the  sides  of  which  are  attached 
the  powerful  thoracic  muscles 
which  depress  and  elevate  the 
fore  limbs ;  an  analogous  cari- 
nation  is  to  be  seen  in  Bats. 
In  the  fossorial  Insectivora, 
such  as  the  mole  (Fig.  146), 
the  presternum  is  produced 
far  forwards,  is  keeled,  and 
widened  out  at  the  sides  so 
as  to  afford  a  large  surface 
of  attachment  for  the  mus- 
cles that  move  their  digging 
limbs. 

In  all  the  gnathostomatous  Yertebrata  the  body 
is  typically  provided  with  two  pairs  of  lateral  ap- 
pendages, or  limbs,  one  of  which  lies,  as  a  rule,  some 
distance  in  front  of  the  other;  these  are  the  fore 
and  hind  limbs.  They  are  brought  into  connection 
with  the  axial  skeleton  by  means  of  arches,  the 
pectoral  and  pelvic  arches. 

The  Pectoral  arch  consists  essentially  of  a  bar  of 
cartilage,  which  undergoes  division  into  a  dorsal  part, 


Fig.  145. — Sternum  of  Fregi- 
lupus  varius. 

cl.  Clavicle  ;  sc,  scapula ;  co,  cora- 
coid  ;  cs,  keel  of  sternum. 
(After  Murie.) 


348  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

or  scapula,  and  a  ventral  portion,  the  coracoid ; 

at  their  point  of  junction  the  proximal  end  of  the  fore 
limb  is  attached.  The  pelvic  arch  is,  similarly,  a 
cartilaginous  bar,  which  may  be  divided  into  a  dorsal 


Fig.  146.— Sternum  of  common  Mole. 


iliac,  and  a  ventral  pubic  piece  ;  and  the  head  of  the 
hind  limb  is  attached  to  the  arch  at  the  point  where 
its  two  halves  unite. 

The  coracoid  divides  into  two  processes,  the 
prsecoracoid  and  coracoid  proper;  the  pubic 
bar  is,  similarly,  in  the  higher  forms,  separable  into 
an  anterior  pufois  and  a  posterior  ischiuiu.  In 


chap,  ix.]  PECTORAL  ARCH.  349 

Fishes,  where  there  is  no  sternum,  the  coracoids  early 
unite  in  the  ventral  median  line  ;  where  a  sternum  is 
present  the  coracoid  of  either  side  enters  into  con- 
nection with  it.  In  the  hind  arch,  where  there  is  no 
bar  corresponding  to  the  sternum,  the  pubes  and  ischia 
may  unite  with  their  fellows  at  a  symphysis,  as  in  all 
Mammalia ;  in  all  Vertebrates  above  fishes,  the  ilia 
enter  into  a  more  or  less  firm  union  with  the  sacral 
region  of  the  axial  skeleton,  or  vertebral  column. 
(See  page  321.) 

Just  as  in  the  case  of  the  skull,  when  covering 
membrane  bones  are  developed  some  enter  into  union 
with  the  cartilage  or  cartilage  bones  of  the  pectoral 
arch,  but  no  such  bones  go  to  form  part  of  the  pelvis. 
The  most  important  of  these  bones  is  the  clavicle, 
with  which  in  some  fishes  a  siipraclavicle,  con- 
nected with  the  skull,  and  an  mfraclavicle,  united 
with  its  fellow  below,  are  added  on ;  in  the  Amniota 
an  interclaviole  is  sometimes  developed.  As  we 
ascend  the  scale  we  observe  a  reduction  in  the  cora- 
coidal  region  ;  thus  the  precoracoid  is  absent  in  the 
crocodiles,  and  among  birds  is  found  in  a  rudimentary 
condition  in  the  Ratitae  only  ;  in  mammals  it  is  never 
present,  and  the  Prototheria  alone  have  a  fully  de- 
veloped coracoid  ;  as  a  rule  the  scapular  end  of  the 
bone  is  alone  retained  ;  it  is,  indeed,  from  the  hook- 
like  form  of  its  remnant  in  man  that  the  bone  has 
received  its  name.  In  a  few  (shrew,  mouse)  the 
sternal  end  of  the  coracoid  is  persistent  (Gegenbaur). 

In  Chamseleons  and  Crocodiles  the  clavicle  is  lost, 
as  it  is  also  in  many  Katitse,  where,  if  present,  it  is 
only  rudimentary  ;  in  the  Carinatse  the  two  clavicles 
unite  to  form  a  single  bone,  the  furcula  ("  merry- 
thought"), which  becomes  connected  by  ligaments 
with  and  strengthens  the  ear  in  a  sterni  (Fig.  145). 
Among  mammals  the  clavicle  may  be  well  developed, 
as  in  man  and  the  bat,  bony  in  its  median  region 


350  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

only,  as  in  the  rabbit,  and  still  more  in  the  dog,  or 
absent,  as  in  bears  and  all  Ungulates.  The  iiiter- 
clavicle  is  developed  only  in  Prototheria. 

In  the  Teleostei  the  pubic  arch  may  be  placed  far 
forwards,  and  be  thoracic,  when  the  hind-fins  lie 
below  the  fore-fins ;  or  jugular,  when  they  lie  in 
front  of  them  (Fig.  134).  In  the  higher  forms  they 
always  retain  their  position  between  the  abdomen  and 
the  tail.  In  recent  reptiles  the  ilium  may  extend 
far  back,  while  in  birds,  and  in  certain  extinct 
reptiles  (Dinosaurs)  it  is  developed  anteriorly  to  the 
acetabulum,  or  cavity  of  articulation  for  the  head  of 
the  hind  limb.  In  the  Sauropsida  this  cavity  is  never 
completely  bony,  and  the  same  is  the  case  in  Echidna 
(though  not  in  Ornithorhynchus)  ;  with  this  exception 
the  acetabulum  is  always  a  completely  bony  cup  in 
mammals. 

In  correlation  with  the  posture  or  mode  of  pro- 
gression, the  ilium  enters  into  more  or  less  close  union 
with  the  sacral  region  of  the  vertebral  column,  and 
the  demands  made  upon  the  axis  for  further  support 
are  responded  to  by  the  fusion  of  presacral  or  post- 
sacral  (or  both)  vertebrae  with  those  of  the  true 
sacrum  to  form  a  solid  piece.  Thus,  in  a  bird 
(Fig.  147)  the  whole  arch  is  of  groat  size,  while  in  the 
Cetacea  it  is  at  most  represented  by  the  ischia. 

Just  as  there  are  very  many  striking  and  sugges- 
tive points  of  resemblance  between  the  fore  and  hind 
arches,  so  are  the  fore  and  hind  limbs  arranged  on 
essentially  similar  principles. 

It  will  be  most  convenient  to  begin  with  what 
obtains  in  the  Amphibia  and  Amniota,  or  the  penta- 
dactyle  Vertebrata.  Either  limb  may  be  divided  into 
three  regions  :  (a)  arm,  fore-arm,  hand  ;  (ft)  thigh,  leg, 
and  foot.  In  the  arm,  and  in  the  thigh,  there  is  a 
single  bone:  (a)  humerus,  or  (ft)  femur;  in  the 
fore-arm  and  leg  two :  (a)  radius  and  ulna ;  (ft) 


Chap,  ix.]  FORE  AND  HIND  LIMBS.  351 

tibia  and  fibula;  the  (a)  hand  (maims)  is  divi- 
sible into  wrist  (carpus),  palm  (metacarpus),  and 
digits ;  the  (0)  foot  (pes),  into  tarsus,  metatar- 
sus, and  digits.  The  digits  are  typically  five  in 
number,  and  consist  of  a  number  of  separate  pieces, 
articulated  on  one  another ;  the  number  of  these 
phalanges  is  inconstant,  but,  as  we  ascend  the 


Fig.  147.—  Side  View  of  the  Pelvis  of  an  Adult  Fowl. 


iZ,  Ilium;  is,  ischium  ;   pft,  puhis;  dl,  dorsal  vertebrae  ;  cd,  caudal  vertebras; 
AW,  acetabulum.    (After  W.  K.  Parker.) 

series,  we  observe   a  reduction  ;  they  are  sometimes, 
though  not  always,  provided  with  horny  claws. 

The  humerus,  which  fits,  by  its  anterior  rounded 
head,  into  the  glenoid  cavity  formed  by  the  edges  of 
the  scapula  and  coracoid,  acquires  a  greater  freedom 
of  movement  when,  as  in  the  Mammalia,  the  coracoid 
is  reduced  in  extent.  It  is  generally  characterised 
by  the  possession  of  a  more  or  less  strong  bony  ridge 
to  which  the  deltoid  muscle  is  inserted,  the  size  of 
which  is,  of  course,  in  proportion  to  the  use  to  which 
the  fore  limb  is  put  ;  and  the  ridge,  therefore,  is  very- 
pronounced  in  fossorial  and  flying  forms.  In  the 
Carinatse  the  muscle  that  elevates  the  wing  lies,  with 
those  that  depress  it,  in  the  pit  formed  by  the  sides  of 
the  keel  of  the  sternum,  and  its  tendon  passes  over 


352  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the   coracoid   and    scapula   to   be   inserted    into   the 
humerus  ;  in  other  words,  it  works  over  a  pulley. 

While  a  flying  organ  is  developed  in  the  Carinatse 
(Fig.  148)  by  the  elongation  of  the  humerus  and 
of  the  radius  and  ulna,  the  latter  of  which  is 
marked  by  the  impressions  of  the  secondary  wing- 
feathers  ;  while  both  are  so  articulated  with  the  lower 
end  of  the  humerus  as  to  allow  of  little  movement 


in 


Fig.  148.— Skeleton  of  Fore  Limb  of  a  Flying  Bird. 
h,  Humerus  ;  r,  radius  ;  u,  ulna  ;  c  c,  carpal  bones ;  ra,  metacarpus  ;  1,  2, 3,  digits. 

on  one  another,  or,  in  other  words,  aid  in  the  forma- 
tion of  a  rigid  rod  ;  by  the  reduction  of  the  carpus 
to  two  bones ;  and  by  the  fusion  of  the  second  and 
third  metacarpals  and  the  reduction  of  the  digits  .  a 
very  different  modification  of  the  homologous  parts 
is  found  in  the  bat.  Here  (Fig.  149)  all  the  meta- 
carpals, save  that  of  the  first  digit  (thumb),  are  greatly 
elongated,  as  are  too  the  phalanges  of  the  third, 
fourth,  and  fifth  digits ;  between  these  and  the  body 
there  extends  a  fold  of  thin  membrane  (the  "wing 
membrane  "),  by  the  expansion  of  which  these  Mam- 
mals are  enabled  to  float  and  move  through  the  air. 


Chap.  IX.] 


FORE  LIMBS. 


353 


In  Galeopithecus  (the  so-called  Flying  Lemur),  the 
fore  and  hind  limbs  are  both  elongated,  and  there 
stretches  between  them,  attached  to  the  sides  of  the 
body,  a  fold  of  skin  which,  unlike 
the  wing  membrane  of  a  bat,  is  hairy 
on  either  side.  Among  lizards  a 
flying  form  is  represented  by  Draco, 
where  the  support  for  the  flying 
apparatus  is  afforded  by  the  elonga- 
tion of  the  ribs. 

Another  set  of  modifications  is 
to  be  found  in  the  aquatic  penta- 
dactyle  forms,  such  as  the  turtles, 
crocodiles,  and  aquatic  mammalia. 
Here  the  essential  modification  con- 
sists in  the  elongation  of  the  manus 
or  pes  to  form  a  fin-like  organ  ;  the 
simplest  and  first  change  is  seen  in 
river  tortoises,  where,  as  in  the  feet 
of  wading  birds,  the  digits  are 
merely  connected  by  a  web ;  as  this 
web  extends,  it  gradually  encloses 
the  separate  digits  and  converts  the 
organ  into  a  paddle,  as  in  the  marine 
turtles,  where  the  fore  are  larger 
than  the  hind  limbs,  or  the  whales 
and  Sirenia,  in  which  the  hind  limbs 
are  altogether  aborted  ;  the  Sirenia 
have  rudimentary  nails,  and  agree 
with  other  mammals  in  never  having 
more  than  three  phalanges  to  their 
digits.  The  whales  present  a  more 
extreme  case,  as  all  rudiments  of 
nails  are  lost,  and  the  phalanges  of  some  of  the  digits 
may  come  to  be  as  many  as  twelve  or  thirteen  in 
number  (Fig.  150). 

In   the    Mammalia   we   observe    tha.t   the    limbs 
x— 16 


Fig.  149.— Manus  of 
Bat. 

p,  Pollex  ;  sc,  scaphoid ; 
mi  to  m*,  the  elon- 
gated metacarpals  of 
the  second  to  fifth 
digits. 


354  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


ordinarily  become  organs  of  support  for  the  body, 
and  that  in  them  more  than  in 
other  Vertebrates  these  ap- 
pendages cease  to  lie  along- 
side the  body,  or  in  a  plane 
more  or  less  parallel  to  its 
long  axis,  and  come  to  be  set 
in  a  vertical  plane.  This 
change  having  been  effected, 
we  note  here,  as  elsewhere  in 
the  organs  of  the  animal  body, 
a  reduction  of  superfluous 
parts,  and  a  consolidation  of 
what  remains.  In  a  large 
number  of  Mammals,  the 
thumb  (pollex)  or  great  toe 
(hallux),  which  never  have 
more  than  two  phalanges,  ex- 
cept in  some,  though  not  all, 
Cetacea,  is  completely  lost. 
Among  the  Ungulata  the 
tendency  to  a  further  reduc- 
tion is  seen  in  the  sheep  and 
ox  on  the  one  hand,  where 
two  digits  persist,  and  in  the 
horse,  where  the  whole  weight 
is  carried  by  the  middle  or 
third  digit  of  each  limb.  The 
historical  evidence  as  to  the 
gradual  reduction  of  the 
second  and  fourth  digits  in 
the  horse  may  be  regarded  as 
complete.  (Compare  Fig.  151.) 
While  reduction  affects 
the  digits,  consolidation  is 
more  often  seen  in  the  metacarpal  and  metatarsal, 
and  carpal  and  tarsal  bones ;  the  muscles  that  are 


igf.  150.  —  Fore  -  arm  and 
Manus  of  the  Round- 
headed  Dolphin. 

Radius  ;  u,  ulna  ;  c,  carpal 
hones  ;  mi,  first ;  ?>i5,  fifth 
metacarpal ;  i  to  v,  digits. 


Chap.  IX.] 


FEET  OF  UNGULATA. 


355 


connected    with    the    lateral    digits    gradually    dis- 
appear. 

In  the  series  of  Artiodactyla  (even-toed  forms) 
we  find,  to  take  the  foot,  four  toes,  distinct  metatarsal, 
and  distinct  tarsal  bones  (in  the  pig) ;  in  the  Chevro- 
taiiis  (Tragulus),  the  second  and  fifth  digits  are  still 
smaller,  and  while  their  metatarsals  are  distinct,  the 
third  and  fourth 
metatarsals  have 
united  together, 
two  of  the  tar- 
sals  have  united 
together,  and 
one  of  the  rest 
has  disappeared; 
in  the  musk- 
deer,  as  in  the 
true  deers,  the 
outer  digits  are 
not  directly  arti- 
culated with  the 
other  bones  of 
the  foot,  and  the 
outer  metatar- 
sals have,  as  in 


Fig.  151. — Foot  of  Anchitherium  (A);   Hippa- 
riou  (B),  and  Horse  (c). 


ii,  in,  iv,  digits. 

them,  disappeared  ;  the  musk-deer,  however,  retains 
what  the  deer  have  lost,  the  extensor  muscle  of  the 
fifth  digit. 

While  the  large  number  of  what  are  really  or 
practically  two- toed  Ungulates  is  evidence  that  this 
reduction  of  the  digits  has  not  been  associated  with 
any  diminution  in  the  value  of  the  limbs  as  locomotor 
or  supporting  organs,  we  have  palaeontological  evidence 
of  the  disappearance  of  a  group  of  even-toed  Ungulates 
who  tended  to  lose  their  lateral  digits.  When  we  ex- 
amine the  carpus  of  a  deer  we  see  that  the  carpal  bones 
have  fused  with  one  another,  and  have  not  disappeared 


356  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


as  the  lateral  digits,  with  which  some  of  them  were 
connected,  have  lost  their  function  ;  as  the  middle  digits 
have  grown  larger  and  thicker  they  have  seized  on  the 
carpal  bones,  and  thereby  gained  "  a  better  and  more 
complete  support  for  the  body."  In  some  fossil  forms 
(Xiphodon,  Anoplotherium),  "  the  relation  between  the 
carpal  and  tarsal  bones,  and  the  remaining  two  middle 
metacarpals  and  metatarsals,  remains  just  the  same  as 
it  was  in  the  tetradactyle  ancestor";  the  digits  that 
remain  do  not,  in  other  words,  gain  further  support 
from  the  carpal  or  tarsal  bones.  Forms  in  which 
inheritance  has  been  stronger  than  modification 
have  disappeared,  while  in  those  which  have  lived  on 
or  left  descendants,  an  adaptive  modification 
has  been  effected  (W.  Kowalevsky). 

As  we  ascend  the  scale  of  the  Primates  we  find 
an  increasing  tendency  to  throw  the  support  of  the 
body  on  the  hind  limbs  only ;  thus,  all  the  manlike 
(anthropomorphous)  apes  are  semi-erect;  the 
Gibbon  (Hylobates)  uses  the  tips  of  his  fingers  much  as 
an  active  man  uses  a  walking  stick  (Huxley),  the  orang, 
the  gorilla,  and  the  chimpanzee,  support  themselves  on 
their  knuckles.  Man  is  erect,  and,  in  correlation  with 
this  position,  the  tuberosity  of  the  os  calcis  of  the  foot 
is  greatly  broadened,  the  thigh  and  leg  are  in  a 
straight  line,  the  pelvis  becomes  an  open  basin  sepa- 
rated by  a  wide  space  from  the  thorax,  the  vertebral 
column  takes  on  a  marked  S  -shaped  or  sigmoid  curva- 
ture, the  head  is  balanced  on  the  atlas,  and  the  spines 
of  the  cervical  vertebrae,  which  have  no  longer  to  give 
origin  to  powerful  muscles,  are  reduced  in  size.  Owing 
to  the  monopoly  of  support  enjoyed  by  the  hind-limbs, 
the  fore  limbs  become  free  to  serve  as  prehensile 
organs,  and  in  man,  where  there  are  no  great  canines 
(as  in  male  gorillas)  to  serve  as  organs  of  attack,  it  is 
to  the  arms  only  that  such  an  animal  can  look  for 
offensive  or  defensive  organs. 


Fig.  152.— Skeleton  of  the  Left  Fore  Limb  of  a  Pig  (A)  ;  Hyomoschus  or 
African  Deerlet  (B)  ;  Tragulus  or  Javan  Deerlet  (c) ;  Roebuck  (D)  ; 
Sheep  (E)  ;  Camel  (F).  (After  Garrod.) 


358  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


While  the  results  of  the  erect  position  show  that 
man  has  been  able  to  adapt  his  altered  mode  of  pro- 
gression to  the  mechanical  conditions  of  an  organisa- 
tion best  suited  for  quadrupedal  movement,  it  is  to  be 
noted  that  (1)  the  space  between  the 
thorax  and  the  pelvis  leads  to,  and  is  the 
cause  of,  prolapse  and  other  affections  of 
the  uterus,  and  of  hernia  in  both  sexes  ; 
(2)  the  carotids  which  supply  the  most 
important  of  organs,  the  brain,  have  to 
carry  their  contents  against  the  action  of 
gravity,  and,  for  this  reason,  they  are  of 
large  size.  Or,  to  put  it  in  another  way, 
the  erect  position  entails  certain  positive 
disadvantages. 

In  other  members  of  various  divisions 
of  Vertebrates,  by  far  the  most  important 
part  in  support  or  locomotion  is  often 
undertaken  by  the  hind  limbs;  this  is 
especially  well  seen  in  hopping  or  jump- 
ing forms,  as,  for  example,  the  frog, 
where  the  tarsal  bones  are  greatly  elon- 
gated and  the  digits  of  considerable 
length;  in  the  jerboa,  where  the  meta- 
tarsals  are  very  long  ;  or  the  kangaroo, 
where  the  calcaneum  (c)  is  very  long,  the 
cuboid  (CB)  very  strong,  and  the  meta- 
tarsal  of  the  fourth  digit  greatly  elon- 
gated ;  in  other  words,  we  have  here  a  continuous 
series  of  well-developed  bones  lying  along  one  axis, 
and  affording  a  firm  support  (Fig.  153). 

When  the  extremities  are  used  as  seizing  organs, 
the  pollex  of  the  maims  and  the  hallux  of  the  pes  are 
qpposable  on  the  other  digits  ;  such  an  arrangement 
obtains  in  the  higher  Primates,  but  in  man,  where  the 
foot  has  more  of  a  supporting  than  of  a  prehensile 
function,  this  power  of  opposition  is  lost  in  many  races 


Pes 


Chap,  ix.]  FINS  OF  FISHES.  359 

by  the  hind  limb,  though  it  can  be  regained  under  the 
stress  of  necessity,  or  by  education  ;  the  saddle-shaped 
form  of  the  articular  surface  of  man's  trapezium  gives 
the  mechanical  reason  for  the  power  of  apposition  of 
the  thumb,  which  he  possesses  in  so  marked  a  degree. 

In  the  tendons  of  the  digits  extra  bones  (sesa- 
moids)  are  not  unfrequently  developed,  and  their 
presence  is  no  doubt  to  be  explained  by  a  refer- 
ence to  the  primitively  multiradiate  condition  (see 
page  361)  of  the  vertebrate  limb;  of  such  bones  the 
most  constant  is  the  patella  (knee-cap),  which  is 
found  in  all  Mammals  save  a  few  Marsupials ; 
another,  which  is  very  frequently  found  in  the  carpus, 
is  the  so-called  pisiform  (or  pea-shaped  bone  of  the 
human  hand).  The  sesamoids  are,  as  will  be  imme- 
diately explained,  most  commonly  developed  in  asso- 
ciation with  the  digits  ;  thus,  in  the  dog  they  are 
found  on  each  metacarpal ;  in  the  fossorial  armadillos 
there  is  a  large  sesamoid  on  the  palmar  side  of  the 
metacarpus  ;  two  large  palmar  sesamoids  are  found  in 
Ornithorhynchus  ;  while  in  the  just-mentioned  Mono- 
treme,  as  to  a  less  extent  in  Echidna  also,  there  is  a 
large  sesamoid  in  the  tarsus  which  supports  the  spur 
of  the  foot,  that  has  so  remarkable  a  likeness  to  what 
is  found  in  the  fowls  and  some  other  birds. 

The  paired  fins  of  Fishes  are,  at  first  sight, 
difficult  to  bring  into  alliance  with  the  pentadactyle 
limb  of  the  higher  Vertebrata.  If  we  take  the  dog- 
fish as  a  type,  we  find  that  the  pectoral  are  larger 
than  the  pelvic  fins,  and  more  complicated  in  cha- 
racter. We  will  commence,  therefore,  with  an  account 
of  the  latter.  They  lie  horizontally,  and  approach 
one  another  at  the  ventral  median  line.  A  long  basal 
bar  (Fig.  154;  A,  bp)  is  articulated  to  a  process  of  the 
ilium,  and  bears  on  its  outer  side  a  series  of  rays,  which 
are  each  divided  into  a  larger  proximal  or  basal  and  a 
smaller  distal  piece,  almost  parallel  to  one  another; 


Fig.  154.— A,  Eight  Pelvic  Fin  and  part  of  Pelvic  Arch  of  an  Adult 
Female  of  ScylUum  canalicula  (nat.  size).  B,  Eight  Pectoral  Fin  and 
part  of  Arch  of  an  Adult  ScylUum  canilicula. 

co,  Coracoid ;  sc,  scapula ;  pp,  protopterygiurn ;  mep,  mesopterygiura ;  mp, 
nietapteryprium ;  il,  iliac  process  ;  pp,  pubic  process,  cut  across  below  ;  bp, 
basipteryprium  ;  /«,  anterior  fin-ray ;  fn,  part  of  fin,  supported  by  horny  fibre. 
CAfter  Balfour,  P.Z.S.,  ]8:-l,  p.  663.)  ' 


chap,  ix.]  FINS  OF  FISHES.  361 

the  most  proximal  articulates  directly  with  the  ilium 
(il),  and  the  most  distal  is,  in  the  male,  converted 
into  the  clasper.  (See  page  519.)  The  outer  portion 
of  the  integument  of  the  fin  is  supported  by  horny 
fibres  (fn).  The  pectoral  fin  (Fig.  154;  B)  is  at  least 
twice  as  large  as  the  pelvic,  and  is  placed  horizontally, 
but  the  two  halves  do  not  approach  one  another  ven- 
trally ;  there  are  three  basal  cartilages,  called  respec- 
tively (Gegenbaur)  protopterygium  (pp),  meso-  (mep), 
and  metapterygium  (mp)  ;  the  latter  carries  most  of 
the  cartilaginous  rays,  and  these  are  divided  into  a 
larger  number  of  pieces  than  the  corresponding  rays 
of  the  pelvic  fin ;  as  with  it,  the  greater  part  of  the  fin 
is  supported  only  by  horny  fibres. 

According  to  the  observations  of  Balfour,  the 
paired  fins  arise  as  ridge-like  thickenings  of  the  epi- 
blast  (see  page  33) ;  the  mesoblast  that  invades  the 
ridge  gives  rise  to  a  cartilaginous  bar,  which,  at  first, 
lies  parallel  to  the  long  axis  of  the  body.  On  one 
side  (the  outer)  of  this  bar  a  thin  plate  extends  out- 
wards, and  this,  by  becoming  divided,  gives  rise  to 
the  primary  fin  rays ;  this  simple  condition  is  essen- 
tially retained  in  the  pelvic  arch;  in  the  pectoral, 
however,  the  basal  bar  becomes  rotated  outwards,  so 
that  it  is  now  only  connected  by  its  anterior  end  with 
the  pectoral  arch,  and  the  bar,  in  place  of  being  the 
basal  portion,  now  forms  the  hinder  border  of  the  fin ; 
the  plate  attached  to  the  bar  becomes  imperfectly 
divided  into  a  smaller  proximal  and  a  much  larger 
distal  piece  ;  from  the  edge  of  each  of  these,  rays  are 
given  off;  the  smaller  piece  undergoes  a  second  divi- 
sion, by  which  we  have,  at  last,  the  protopterygium 
(pt)  with  one  ray,  and  the  mesopterygium  with 
a  few  ;  the  rest  of  the  rays  are  attached  to  the  meta- 
pterygium, or  larger  distal  piece. 

On  the  supposition  that  a  many-rayed  limb  of 
the  characters  just  described  is  that  from  which  the 


362  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

pentaclactyle  limb  of  the  higher  Vertebrata  has  been 
developed,  we  must  suppose  that  the  greater  number  of 
the  cartilaginous  pieces  have  undergone  reduction,  and 
that,  in  the  Ichthyosauria  for  example,  or  in  the  frog, 
where  there  is  a  rudiment  of  a  sixth  digit  to  the  foot,  the 
number  five  is  exceeded  in  consequence  of  the  re- 
duction not  having  been  definitively  impressed  on  the 
organism  by  inheritance ;  on  the  other  hand,  the 
possession  by  a  Mammal  (e.g.  man)  of  more  than  five 
digits  (polydactylism)  must  be  regarded  rather  as  an 
abnormality  than  as  a  return  to  an  ancestral  condi- 
tion, and  this  because  the  gap  between  a  man  and  a 
polydactylous  ancestor  is  too  wide  for  us  to  be  able 
reasonably  to  believe  in  an  "  atavism  "  so  far-reaching. 

In  the  Ganoidei  or  Teleostei,  the  pterygial  portions 
of  the  fins  are  reduced,  but  the  reduction  is  atoned 
for  by  the  replacement  of  the  horny  fibres  by  osseous 
tissue.  In  Ceratodus  the  fin  takes  the  form  of  a 
central  axis  of  cartilaginous  pieces,  with  rays  on  both 
sides  ;  and  in  Protopterus  it  becomes  filamentar,  owing 
to  the  loss  of  the  lateral  rays.  Gegenbaur  regards  the 
fin  of  Ceratodus  as  the  most  primitive  arrangement 
(archipterygium) ;  but,  as  Balfour  has  pointed  out,  this 
view  of  the  matter  is  opposed  by  the  facts  that  in 
Elasmobranchs  there  are  indications  of  rays  on  one 
side  only  of  the  basipterygium,  and  that  the  support- 
ing bar  is,  at  first,  basal,  and  not  central. 

Like  the  limbs  of  higher  Vertebrates,  the  fins  of 
fishes  are,  at  first  and  in  most  cases,  locomotor  in 
function,  wherein  they  are  aided  by  the  tail ;  just  as 
the  former  are  supporting  organs,  so,  too,  are  the  fins. 
This  may  be  seen  by  removing  the  fins  of  one  side, 
when  the  fish  falls  on  to  that  side ;  or  by  cutting  off 
both  pectorals,  when  the  body  inclines  forwards  and 
downwards.  In  mud-dwelling  fishes  the  pelvic  fins 
are  rudimentary  or  absent,  disuse  producing  degrada- 
tion. One  of  the  most  remarkable  modifications  of 


chap,  ix.i  EXTERNAL  SKELETON.  363 

the  fins  is  seen  in  Periophthalmus,  which,  thanks 
especially  to  its  large  pectorals,  is  able  to  hop  over  the 
mud.  In  some  Gobies  the  ventral  fins  unite  to  form 
a  kind  of  suctorial  disc,  by  means  of  which  the  fish 
can  attach  itself  to  rocks.  The  sucking  disc  of 
Cyclopterus  lumpus  is  supported  by  the  rudimentary 
spines  and  rays  of  the  ventral  fins.  In  the  flying- 
fish  (Exocoetus)  the  pectoral  fins  may  extend  as  far 
back  as  the  caudal,  and  can  be  spread  out  so  as  to 
act  like  sails.  In  cartilaginous  fishes,  where  the 
edges  of  the  fins  are  softer  than  in  the  bony  fishes, 
these  edges  may  perform  an  undulatory  or  screw- 
like  movement.  When  the  lateral  fins  disappear, 
the  locomotor  function  falls  altogether  on  the  vertebral 
column  and  unpaired  fins. 

The  External  skeleton  of  Vertebrates  is,  in 
the  simpler  conditions,  formed  by  scales,  which  are 
developed  in  the  cells  of  the  integument.  The  most 
generalised  condition  obtains  among  Elasmobranchs, 
where,  as  we  have  already  learnt,  the  internal  skele- 
ton is  throughout  life  cartilaginous ;  in  such  a  form 
as  the  dog-fish  the  whole  of  the  external  surface  is 
roughened,  owing  to  the  presence  of  projecting 
pointed  processes,  which  have  not  inappropriately 
been  called  dermal  denticles,  so  close  and  strik- 
ing is  their  resemblance  to  the  processes  which,  when 
placed  within  the  area  of  the  mouth,  are  called  teeth ; 
like  them,  they  consist  essentially  of  dentine  invested 
in  a  layer  of  harder  enamel.  In  the  huge  basking 
shark  the  whole  of  the  body  is  covered  by  denticles, 
which,  taken  separately,  are  small  enough,  but  which 
en  masse  must  be  a  very  effective  means  of  defence. 
In  the  spinous  shark  (Echinorhinchus)  the  diffused  ar- 
rangement yields  to  "one  in  which  large  spinous  tuber- 
cles are  scattered  over  the  body,  and  the  value  of  that 
diffused  arrangement  is  very  eloquently  spoken  to  by 
the  naked  body  of  the  torpedo,  which  has  found  a  still 


364  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

better  mode  of  protection  in  its  well-developed  electric 
organ.  In  various  Elasmobranchs  the  more  prominent 
fins  are  provided  with  strong  ^spines. 

The  Ganoidei  received  their  name  from  the  posses- 
sion by  some  of  them  of  bright  shining  scales,  which 
owe  their  appearance  to  the  investing  layer  of  enamel. 
Such  "ganoid  scales"  are,  however,  found  in 
perfection  only  in  Lepidosteus  and  Polypterus  among 
recent  members  of  the  group;  in  the  sturgeon,  for 
example,  there  are  bony  plates,  and  Spatularia  is 
naked.  In  the  two  Ganoids  first  mentioned  the 
scales  overlap,  and  the  whole  body  is  protected  by 
a  closely  and  firmly  set  coat  of  mail.  Among  fossil 
forms  we  find  the  typical  ganoid  arrangement  some- 
times carried  to  a  remarkable  extreme,  as  in  Pterich- 
thys,  where  large  bucklers  are  found  not  only  on  the 
dorsal  but  also  on  the  ventral  surface.  The  allied 
Dinichthys  is  thought  to  have  reached  a  length  of 
more  than  fifteen  feet ;  and  we  see  in  it,  as  in  other 
gigantic  forms,  such  as  the  Irish  elk,  that  individual 
protection  has  been  only  attained  at  the  cost  of  the 
disappearance  of  the  species. 

The  simpler  smaller  scales  that  are  found  in  some 
Ganoids,  and  very  commonly  among  the  Teleostei, 
may  be,  when  we  look  at  extremes,  classed  under 
the  head  of  cycloid  scales,  in  which  the  free  pro- 
jecting margin  is  rounded,  or  as  ctenoid,  in  which 
the  margin,  or  part  of  the  surface,  is  denticulated  or 
comb-like ;  between  these,  however,  there  are  a 
number  of  intermediate  stages ;  the  ctenoid  scales  may 
be  supposed  to  have  given  rise  to  those  in  which  part 
of  the  surface  is  continued  into  fine  non-denticulated 
spines  (sparoid  scales  of  sea-breams).  While  some 
fishes,  such  as  Stomias,  have  the  scales  deciduate, 
and  others,  not  to  speak  of  electric  forms,  are, 
like  the  eel,  scaleless,  the  Teleostei,  almost  as  much 
as  the  Ganoids,  present  us  with  examples  of  forms  in 


chap,  ix.]  EXTERNAL  SKELETON.  365 

which  the  whole  or  the  greater  part  of  the  body  is 
amply  provided  with  a  defensive  armature.  Such 
forms  are  Osteoglossum  and  its  allies,  in  which  the 
body  is  closely  covered  with  hard  scales,  the  "  cofier- 
fishes "  (Ostracion),  where  the  hexagonal  scales  fit , 
like  the  pieces  of  a  mosaic,  or  the  globe-fishes  (Diodon), 
where  the  whole  of  the  globular  body  is  covered  by 
projecting  and  movable  spines,  which,  standing  out 
on  erection,  must  most  effectually  protect  their  pos- 
sessor. 

The  protective  function  of  the  exoskeleton  of  the 
true  fishes  is  replaced  in  the  Cyclostomata  by  the 
rich  supply  of  mucous  glands  to  the  integument ;  in 
the  hag  this  power  is  carried  to  so  great  an  extreme 
that  a  single  example  placed  in  more  than  three  cubic 
feet  of  water  is  able  to  shed  out  so  much  mucus  that 
the  whole  becomes  converted  into  a  continuous  viscid 
mass;  with  this  power  of  emitting  a  sticking  secretion  we 
may  compare  the  "  cotton-spinner,"  where,  however, 
no  observations  have  yet  been  made  as  to  the  amount 
of  the  secretion. 

The  Amphibia  best  known  to  us  have  a  soft 
unarmed  integument,  but  the  Csecilise,  among  recent 
forms,  have  small  cycloid-like  scales  in  their  integu- 
ment, a  few  Urodeles  have  flat  bony  plates,  and  the 
extinct  Labyrinthodonta  would  seem  to  have  had  a 
plentiful  supply  of  well-developed  ventral  plates. 

Among  the  Reptilia  we  have  thickenings  which 
may  merely  form  epidermic  scales*  as  in  snakes  and 
lizards,  or  larger  bony  plates  (scutes),  as  in  croco- 
diles, or  very  extensive  pieces,  as  in  tortoises  and 
turtles.  In  the  Ophidia  the  separate  scales  are  held 
together  by  the  continuous  epidermic  covering  to 
which  they  owe  their  origin,  and  the  whole  is  ordina- 
rily shed  in  one  piece  ;  the  most  remarkable  modifica- 
tion undergone  by  them  is  to  be  seen  in  the  rattlesnake, 
where  the  cuticular  scales  at  the  hinder  end  of  the 


366  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

body  are  converted  into  transversely  oblong  plates, 
which,  when  moved  rapidly  on  one  another,  give  out  a 
rattling  or  vibrating  sound.  Lizards  may  be  scaleless, 
like  the  chamseleon,  which  possibly  makes  up  for  the 
absence  of  the  protective  covering,  such  as  it  is,  by 
the  power  of  so  adapting  itself  to  the  coloration  of 
the  neighbourhood  in  which  it  finds  itself  as  to  be 
almost  invisible ;  or  the  Amphisbaena ;  or  there  may 
be  thin  scales,  as  in  the  true  lizards  ;  or,  as  in  Cyclodus, 
bone  may  be  deposited  in  the  dermis,  and  the  bony 
plates  may,  as  in  the  skink,  unite  into  a  mosaic-like 
arrangement. 

In  the  Crocodilia  there  are  scutes  as  well  as  scales ; 
that  is  to  say,  the  dermis  undergoes  ossification ;  and 
the  separate  scutes  are  covered  by  an  epidermic  thick- 
ening or  scale.  In  a  few  (as  the  Caiman),  the  scales 
on  both  the  upper  and  lower  surfaces  become,  respec- 
tively, so  united  with  their  neighbours  as  to  give  rise 
to  a  dorsal  or  a  ventral  shield ;  on  the  long  tail  the 
upper  and  lower  ossifications  unite  to  form  continuous 
rings.  In  most  crocodiles,  however,  the  ventral 
shields  are  absent,  and  the  dorsal  scutes  do  not  unite 
with  one  another  to  form  such  continuous  pieces  as 
can  properly  be  called  shields. 

The  differences  between  the  horny  epidermic 
hardenings  and  the  osseous  dermal  thickenings  are 
best  exhibited  by  the  Chelonia,  where,  as  is  well 
known,  large  continuous  pieces,  both  of  shell  and  bone, 
are  ordinarily  exceedingly  well  developed.  The 
thinner  epidermic  plates  form  the  so-called  tortoise- 
shell,  the  thicker  dermal  bones  the  plates  of  the 
shield,  or  carapace,  which  enter  into  close  connec- 
tion with  parts  of  the  endoskeleton. 

In  Birds,  the  outer  covering  is  in  the  form  of 
feathers ;  a  feather  consists  of  a  central  quill,  shaft, 
or  scapus  (Fig.  155;  d),  from  which  on  either  side 
there  are  given  off  flattened  branches,  or  barbs; 


Chap.  IX.] 


FEA  THERS. 


the  latter  similarly  give  off  much  finer  radii  or 
barbules,  which,  interlocking  by  "cilia"  and  booklets 
with  those  that  are  found  on.  neighbouring  barbs, 


c/ 


Fig.  155.— Feather  from  the  Back  of  Argus  giganteus. 

a,  Shaft  (rachis) ;  6,  aftershaft :  c,  branches  to  forna  the  vexillum,  removed 
from  one  side  of    both   shaft  and   undershaft ;  d,  shaft  (scapus).     (After 


form  the  connected  vane  or  vexillum  of  the  feather 
(c)  ;  the  shaft,  which  in  its  upper  portion  is  often 
called  the  rliachis,  frequently  gives  off  near  its  base 
a  smaller  feather  or  aftershaft  (b}.  It  has  been 
calculated  (by  Gadow)  that  the  feather  of  an  eagle 
contains  about  two  thousand  barbs,  five  millions  and  a 


368  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


half  of  barbules,  and  fifty-four  millions  of  cilia  and 
booklets.  These  feathers  are  not  irregularly  arranged, 
but  are  set  along  definite  tracts  (feather  tracts)  the 

arrangement  of  which 
(pterylosis)  varies  in 
various  birds,  and  has, 
since  the  time  of  Nitzsch, 
been  made  use  of  in 
classification  (Fig.  156). 
The  function  of 
feathers  is  not  limited 
to  the  diminution  of 
the  specific  gravity  of 
the  bird,  which  they 
effect  by  entangling  air ; 
the  same  process  is  also 
of  aid  in  preserving  the 
high  temperature  of 
these  creatures,  in  con- 
sequence of  the  feeble 
conductive  power  of  air. 
So  far  as  the  former 
effect  is  concerned,  we 
have  to  note  that  the 
Ratite  birds,  which 
never  soar  into  the  air, 
are  without  the  barbules 
by  means  of  which  the 
barbs  form  a  connected 
vane. 

The  hairs  of  Mam- 
mals, like  the  feathers 

of  birds,  are  epidermic  in  origin,  but  their  mode  of 
development  is  somewhat  different.  As  a  general 
account  of  the  structure  of  hair  has  already 
been  given  in  chap,  xxxiv.  of  Klein's  "  Elements  of 
Histology,"  it  is  here  only  necessary  to  give  some 


Fig.  156.— Pterylosis,  or  arrangement 
of  Feather-tracts  on  the  under 
surface  of  the  body  of  a  Cock 
(Gallus  bmikiva).  (After  Nitzsch.) 


Chap,  ix.]  HAIRS  OF  MAMMALS.  369 

account  of  their  arrangement  in  different  forms. 
Hair  is  almost  entirely  absent  from  the  body  of  adult 
Cetacea,  and  only  scantily  developed  in  the  Sirenia ; 
this  common  character  must  not,  however,  be  regarded 
as  any  evidence  of  community  of  origin  or  closeness 
of  relationship,  but  rather  as  the  result  of  exposure 
to  similar  conditions.  Sometimes,  when  the  hair  is 


Fig.  157.— The  Armadillo. 

scanty  on  the  body,  as  in  the  rhinoceros,  a  number  of 
hair-like  shafts  unite  to  form  a  horn.  In  forms  which 
live  in  very  cold  climates,  like  the  musk-sheep  (Ovibos 
moschatus),  the  hair  is  exceedingly  long  and  thick,  and 
serves  as  an  efficient  protection  against  the  external 
cold ;  the  most  striking  example  of  this  is  afforded  by 
the  thick  coating  of  the  extinct  mammoth,  which 
lived  in  cold  regions,  whereas  its  allies,  the  elephants, 
which,  in  recent  times,  are  confined  to  warm  coun- 
tries, have  but  little  hair.  The  soft  hair  may  be 
replaced  by  firm  and  strong  spines,  as  in  the  porcu- 
pine or  hedgehog,  where,  thanks  to  their  power  of 
Y— 16 


37 o  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

erection,  they  form  very  efficient  organs  of  defence 
and  protection. 

Sometimes  the  hairs  become  specially  endowed 
with  a  tactile  function,  as  in  the  "  whiskers  "  of  feline 
and  especially  nocturnal  carnivora ;  reminding  us  so 
far  of  the  elongated  delicate  filaments  of,  no  doubt, 
similar  functions,  which  are  found  on  the  bodies  of 
deep-sea  fishes. 

The  hairs  may  be  greatly  elongated,  and  used,  as 
in  horses,  for  switches,  by  means  of  which  their  bodies 
are  freed  from  offending  insects. 

The  claws  found  on  the  digits  of  various  lower 
Vertebrates  are,  as  "  nails,"  almost  constantly  present 
in  Mammals,  where  they  may  be  flat,  as  in  man,  sharp 
offensive  claws,  as  in  Carnivora,  large  protecting  hoofs, 
as  in  Ungulates,  or  organs  of  support  to  arboreal 
forms,  such  as  the  bat  or  the  sloth ;  they  are  wanting 
in  the  Cetacea. 

The  only  Mammals  in  which  long  dermal  scutes 
are  now  developed  are  the  armadillos  (Fig.  157),  where 
three  or  more  zones  may  be  present,  and  form  a  more 
or  less  complete  protective  covering  for  these  animals  ; 
such  scutes  were  present  in  enormous  numbers  in  the 
extinct  Glyptodon  and  Hoplophorus. 


CHAPTER  X. 

ORGANS    OF    MOVEMENT. 

IN  the  Protozoa,  where  division  of  labour  never 
proceeds  so  far  as  to  lead  to  the  formation  of  definite 
tissues,  the  function  of  locomotion,  like  all  the  rest, 
is  simply  performed  by  the  protoplasm  of  the  cell, 
which,  as  we  have  already  learnt,  is  contractile. 
Thanks  to  this  power  of  contractility,  even  an 


Chap,  x.j          MOVEMENTS  OF  PROTOZOA.  371 

amorphous  mass  like  an  Amoeba  is  enabled,  by  the 
withdrawal  of  one  and  the  protrusion  of  another  part 
of  its  substance,  to  move  about  from  place  to  place.  In 
the  ciliated  forms  movement  is  due  to  the  contractile 
action  or  play  of  those  delicate  processes  of  proto- 
plasm which  form  the  cilia  ;  between  the  fine  processes 
that  we  ordinarily  call  by  that  name,  and  the  coarser, 
more  lobate,  processes  that  are  distinguished  as  pseudo- 
podia,  the  connection  is  very  close,  and  under  certain 
conditions  one  form  may  be  observed  to  pass  into  the 
other.  Among  certain  stalked  Infusoria,  such  as 
Vorticella,  we  observe  a  mode  of  movement  which  is 
more  rapidly  executed  than  that  of  ordinary  trans- 
lation ;  a  Vorticella,  or  its  branched  ally,  Carchesium, 
may  be  seen  to  suddenly  lower  its  bell,  owing  to  the 
rapid  contraction  in  length  of  its  stalk  ;  the  agent  by 
which  this  is  effected  is  a  modified  portion  of  the 
protoplasm  in  the  stalk  (the  so-called  contractile  band), 
which  presents  a  striation  that  calls  to  mind  that  of  a 
muscular  fibre.  Though  agreeing  with  it  functionally, 
the  stalk  differs  from  it  morphologically,  in  that  it 
is  a  modification  of  only  part  of  a  cell,  and  not  of  a 
whole  cell,  or  of  a  set  of  cells.  These  bands  are  not 
confined  to  the  stalked  Infusoria,  but  are  found  in 
other  forms  both  of  the  Ciliata  and  of  the  Gregarinida; 
without  •  them,  indeed,  there  can  be  but  feeble  move- 
ments in  the  latter  endoparasitic  organisms,  which  are 
without  either  cilia  or  pseudopodia. 

Among  the  lower  Metazoa  we  find  that  the 
movements  of  the  young  are  at  first  effected  not  by 
muscular  tissue,  but  by  cilia  ;  the  free-swimming  larva 
being  provided  with  cilia,  which  may  be  scattered 
over  the  whole  of  the  body,  or  confined  to  certain 
definite  and  characteristic  tracts,  such  as  circlets,  one 
or  more  in  number,  or  wavy  bands  (Fig.  158).  In  all 
groups,  save  that  of  the  Porifera,  the  cilia  are  found 
on  the  outer  surface  of  the  body  or  epiblast,  and  in 


372  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


all  but  it,  the  members  of  which  are  always  fixed 
when  adult,  a  definite  tissue,  or  collection  of  cells, 
becomes  specially  endowed  with  a  contractile  function, 

and  forms  muscular 
tissue,  and  a  more 
or  less  regularly 
disposed  muscular 
system.  (For  the 
minute  structure  of 
muscle  see  "  Klein's 
Histology,"  chaps, 
viii.  and  ix.) 

In  Hydra,  among 
the  Ccelenterata, 
the  only  indications 
of  muscular  tissue 
are  the  branched 
prolongations  in- 
wards of  certain  of 
the  cells  of  the 
ectoderm  (neuro- 
muscular  cells  of 
Kleinenberg,  or, 
more  shortly,  Klei- 
nenberg's  cells) ;  in 
it  the  several  cells 
of  the  body  still  re- 
tain their  indepen- 
dent contractility. 
In  higher  forms  the 
epithelial  ingrowths 
become  more  independent,  and  in  the  Medusae  they 
become  transversely  striated.  In  these  last  they  form 
a  sheet  on  the  lower  face  of  the  disc  or  umbrella,  which 
in  living  specimens  is  repeatedly  opening  and  closing  ; 
they  are  continued  into  the  tentacles,  and  when  a 
velum  is  present  they  are  largely  developed  in  it. 


Fig.  158. — Larva  of  Holothuria  tubulosa  in 
its  natural  position. 

The  arrow  indicates  the  axis  of  rotation,  and 
the  cilia  are  seen  to  be  arranged  in  a  sinuous 
band.  (From  Carpenter,  after  Selenka.) 


Chap,  x.]     MOVEMENTS  OF  CCELENTERATA.  373 

In  the  Actiniae  it  is  possible  to  distinguish  a  system 
of  longitudinal  from  one  of  transverse  muscular  fibres, 
and  the  presence  of  these  two  explains  how  it  is  that  a 
sea-anemone  is  able,  when  irritated,  to  diminish  both 
in  length  and  breadth ;  the  longitudinal  muscles  are 
the  best  developed,  and  may  be  seen  to  be  arranged  in 
definite  bundles;  the  transverse  are  strongest  in  the 
region  of  the  base  of  the  polyp  (Fig.  54).  The  ten- 
tacles owe  their  contractility  to  the  possession  of 
muscular  fibres. 

In  the  Ctenophora,  which  retain  an  external 
investment  of  cilia  along  the  lines  of  their  "  cteno- 
phoral  plates,"  the  greater  part  in  the  production  of 
movements  of  the  body  is  not  effected  by  the  muscles, 
which  are  poorly  developed  in  the  ectodermal  layer, 
but  by  the  contractile  fibres  which  are  developed  in 
the  mesoderm,  which  is  so  richly  developed  in  the 
Ctenophora ;  as  seen  in  Beroe,  these  muscles  are  long 
cylindrical  cords,  which  are  not  united  into  bundles, 
and  are  disposed  radially,  circularly,  and  longitudinally. 
The  greater  number  are,  like  Cydippe  (Fig.  15),  pro- 
vided with  a  pair  of  long  tentacles,  in  addition  to  which 
other  smaller  or  secondary  tentacles  may  also  be 
present ;  the  axis  of  these  is  occupied  by  a  cord  of 
muscular  fibre;  their  most  important  office  is,  no 
doubt,  not  one  of  locomotion,  which  is  effected  chiefly 
by  the  ciliated  paddles,  but  of  prehension,  for  where 
they  are  absent,  the  mouth  is  much  wider  than  it  is 
in  those  that  possess  them. 

Many  of  the  lower  Worms  move  by  the  elonga- 
tion of  the  anterior  end  of  their  body,  which  is  suc- 
ceeded by  a  contraction  by  means  of  which  the  hinder 
part  is  brought  to  its  original  point  of  distance  from 
the  anterior ;  in  the  performance  of  this  operation 
they  are  sometimes  aided  by  one  or  more  cup-shaped 
suckers,  by  means  of  which  a  fixed  point  is  gained  ; 
others,  like  the  leech,  fix  themselves  by  their  hinder 


^^COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

sucker,  and  sway  about  or  elongate  their  body  so  as 
to  reach  their  prey.  In  the  flat- worms  and  in  the 
leeches  there  are  longitudinal,  circular,  and  transverse 
muscles.  In  all  the  rest  of  the  Annulata,  and 
in  the  Gephyrea,  there  are  only  circular  and  longitu- 
dinal bands  in  the  body  wall,  the  former  of  which  are 
the  more  external ;  but,  in  addition  to  these,  there  are 
smaller  muscles  which  are  of  considerable  importance 
in  locomotion,  as  they  are  inserted  into  the  base  of 
the  setse,  and  are  the  means  by  which  these  processes 
are  moved  forwards  and  backwards,  or  used  as  parts  of 
a  locomotor  apparatus,  working  either  as  mere  stilts  as 
in  the  earthworm ;  or,  as  in  the  Polychneta,  where  they 
are  numerous,  like  oars  in  the  free-swimming  forms, 
and  as  climbing  hooks  in  those  that  live  in  tubes. 

Among  the  Echinodemiata  the  most  impor- 
tant organs  of  movement  are  the  contractile  tube  feet, 
which  are  most  valuable  when,  as  often  happens  in 
the  Starfishes,  or  the  Urchins,  they  are  provided  at 
their  free  ends  with  a  sucker-shaped  enlargement  by 
means  of  which  they  can  gain  certain  fixed 
points  to  which  they  can  draw  their  bodies.  When 
climbing  up  vertical,  or  almost  vertical,  heights,  the 
Echinoderin  converts  its  pedicellarise  (see  page  297), 
which  are  provided  with  special  muscles,  into  organs 
of  locomotion,  in  so  far  that  these  pedicellarise  seize 
hold  of  waving  fronds  of  sea-weed,  which  act,  there- 
fore, like  the  rungs  of  a  ladder,  up  which  one  is 
climbing  by  the  use  of  the  hands  only ;  it  is  of  parti- 
cular interest  to  observe  that  "  the  wonderfully  tena- 
cious grasp  of  the  forceps  is  timed  as  to  its  duration 
with  an  apparent  reference  to  the  requirements  of  the 
pedicels  (tube  feet),  for  after  lasting  about  two 
minutes,  which  is  about  the  time  required  for  the 
suckers  (tube  feet)  to  bend  over  and  fix  themselves 
to  the  object  held  by  the  pedicellarise,  if  such  should 
be  a  suitable  one,  this  wonderfully  tenacious  grasp 


chap,  x.]      MOVEMENTS  OF  ARTHROPODA.  375 

is  spontaneously  released "  (Romanes  and  Ewart). 
Ordinary  Ophiuroids,  which,  according  to  the  authors 
just  qiioted,  are  able  to  move  along  at  the  rate  of  six 
feet  a  minute,  have  a  certain  wriggling  power  of 
their  arms,  which,  in  the  Astrophytidse,  is  converted 
into  a  power  of  coiling  for  the  purposes  of  attach- 
ment, thanks  to  the  fact  that  the  faces  of  every  one 
of  their  arm  joints  are  convex  in  one  direction,  and 
concave  in  that  which  is  at  right  angles  to  it.  When 
the  spines  are  long,  as  in  the  piper  (Dorocidaris), 
where  they  are  also  of  considerable  stoutness,  or  in 
Spatangus,  where  they  are  much  more  delicate,  they 
can  be  used  as  stilts,  owing  to  the  attachment  of 
muscular  tissue  to  their  bases. 

In  the  Artliropoda  the  function  of  locomotion, 
like  so  many  other  functions  in  that  group,  falls  very 
largely  upon  the  appendages,  which  may  either  act  as 
walking  or  as  swimming  organs.  In  the  Crustacea, 
where  all  but  the  first  pair  are  typically  biramose, 
this  locomotor  function  is  seen  in  the  early  Nauplius 
condition  (see  page  534),  when  even  the  antennae  take 
part  in  performing  that  duty ;  these  appendages,  being 
jointed  and  provided  internally  with  muscles,  are  able 
to  move  in  various  directions.  At  first,  and  in  the 
lower  forms,  they  act  more  or  less  like  oars,  beating 
the  water  as  they  move  backwards  and  forwards.  In 
the  higher  forms,  such,  for  example,  as  the  crayfish, 
the  more  anterior  of  the  locomotor  appendages  act  as 
walking,  and  the  more  posterior  as  swimming  organs. 
In  an  appendage,  which,  has  been  but  little 
modified,  and  which  may  be  regarded  as  typical,  such 
as  the  pair  formed  on  the  third  abdominal  segment, 
we  see  a  doubly-jointed  basal  piece  or  protopodite, 
bearing  two  terminal  pieces,  the  outer  exopodite  and 
the  inner  endopodite.  These  pieces,  which  are  fringed 
with  long  bristles,  or  setae,  are  flattened,  and  can  act 
like  oars. 


376  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Those  appendages  of  the  thoracic  region  which  are 
confined  to  a  locomotor  function  have  the  form  of  an 
elongated  jointed  bar,  consisting  of  seven  joints,  which 
have  received  the  following  names :  the  first  two, 
which  appear  to  correspond  to  the  protopodite  of  the 
typical  appendage,  are  called  coxopodite  and  basi- 
podite  ;  the  remaining  five,  which  may  be  supposed  to 
represent  the  endopodite,  are 
ischio,  mero,  carpo,  pro,  and 
dactylo-podites. 

While  these  ambulatory 
appendages  move  in  a  plane 
which  is  parallel  to  the  long 
axis  of  the  body,  those  of  the 
abdomen  swing  backwards  a  ad 
forwards,  owing  to  the  fact 
that  the  abdominal  segments 
are,  unlike  those  of  the  thorax, 
which  is  invested  by  the  hinder 
part  of  the  great  carapace, 
capable  of  being  moved  on  one 
another.  This  movement  of 

segments  is  brought  about  by  two  great  bundles  of  the 
muscle,  which  lie  respectively  above  and  below 
the  intestine,  and  are  attached  to  the  tergal  and 
sternal  plates  of  the  separate  segments.  It  follows, 
from  their  mode  of  attachment,  that  the  contraction 
of  the  upper  muscles  straightens  out  or  extends  the 
abdomen  (tail),  and  that  a  contraction  of  the  lower 
muscle  tends  to  bend  in  or  flex  the  same  parts.  An 
alternate  contraction  and  relaxation  of  these  muscles 
tends  therefore  to  an  alternate  bending  in  and 
straightening  out  of  the  "  tail,"  and  therefore  to 
repeated  beats  of  the  water,  by  means  of  which  the 
crayfish  or  lobster  is  driven  through  it.  In  the 
performance  of  this  locomotor  action  the  "  tail "  is 
greatly  aided  by  the  modification  of  the  appendages  of 


Fig.  159.— Third  Abdominal 
Segment  of  the  Lobster. 

f,  Tergura  (dorsal  piece) ;  «, 
sternum  (ventral  piece);  pi, 
pleuron  ;  p,  protopodite  ;  ex, 

•   exopodite ;  en,  endopodite. 


Chap.  x.].          -LEGS  OF  ARTHROPODA.  377 

the  penultimate  segment  of  the  body,  which,  in  place 
of  being  comparatively  small  parts,  as  in  the  typical 
third  abdominal  appendage,  are  widened  out  into 
more  considerable  plates,  which  have  a  backward 
instead  of  a  downward  direction ;  these  unite  with 
the  terminal  segment,  which  sometimes,  though  very 
rarely  (Scyllarus),  bears  minute  appendages,  to  form  the 
powerful  flapper  of  the  Crayfish. 

Peripatus,  the  species  of  which  vary  considerably 
in  the  number  of  appendages,  have  these  organs  only 
imperfectly  jointed,  and  they  move  but  slowly  ;  in 
them,  as  in  all  Arthropods  other  than  the  Crustacea, 
the  limbs  are  uni-,  and  not  bi-ramose,  but,  as  often 
happens,  they  are  provided  with  a  terminal  claw. 

The  Myriopoda  (Centipedes),  as  their  name 
indeed  implies,  have  a  large  number  of  walking  limbs, 
each  of  which  has  essentially  the  same  characters  as 
that  which  precedes  and  that  which  follows  it ;  in 
the  Millipedes  a  number  of  segments  carry  two  pairs 
of  legs  each.  The  Araclmida  have  four  pairs  of 
walking  limbs,  which  are  completely  lost  in  such 
endoparasitic  forms  as  Pentastomum.  The  Insects,  or 
as  they  are  very  appropriately  called,  the  Hexapoda, 
have  three  pairs  of  walking  limbs  ;  these  are  typically 
composed  of  a  coxa,  a  trochanter,  a  femur,  a  tibia, 
and  a  six-jointed  tarsus,  which  ends  in  a  pair  of  claws ; 
the  larval  or  caterpillar  forms  have,  however,  a  more 
or  less  larger  number  of  walking  appendages,  or  pro- 
legs  ;  these  are  best  and  most  numerously  developed 
among  the  Lepidoptera,  but  they  are  in  all  cases 
rudimentary  as  compared  with  the  legs  of  the  adult. 

A  large  number  of  insects  have  yet  another  set  of 
locomotor  organs,  in  the  shape  of  the  dorsally-placed 
wings  ;  of  these  there  are  never  more  than  two  pairs, 
and  of  these  both  may  be  rudimentary,  as  in  the 
female  cockroach ;  or  the  anterior  pair  only  may  be 
developed  as  in  the  Diptera  (flies),  or  the  hinder  alone 


378  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

be  functional  as  in  the  Coleoptera  (beetles).  In  no 
case  are  they  developed  except  on  the  second  and 
third  segments  of  the  thorax  (rneso-  and  meta- 
thorax). 

A    wing   consists    essentially   of    two   flattened 
A 

81 


Fig.  160.— Skeleton  of  Butterfly's  Wing. 

A,  Costalvein ;  B,  subcostal ;  mi  m*,  median  branches  ;  ri  7-2,  radial ;  si  s5,  subcostal 
branches;  c,  median  vein  ;  D,  submedian  vein  ;  dc.  discocellular  veinlet ; 
in,  internomedian  veinlet ;  B,  internal  vein.  (After  Butler.) 

membranes,  the  presence  of  which  is  due  to  the  organ 
arising  in  the  form  of  a  sac,  which  gradually  becomes 
elongated  and  flattened  out ;  through  its  substance 
there  pass  blood-vessels  and  air  tubes,  the  walls  of 
which  are  strengthened  by  chitin ;  the  chitin  may 
invade  the  rest  of  the  wing,  and  convert  it  into  a  more 


chap,  x.]  WINGS  OF  INSECTS.  379 

or  less  horny  body ;  this  process,  when  carried  to  an 
extreme,  ends  in  the  stout  wing-covers  (Elytra)  of  the 
beetle.  These  tracheal  tubes  are  the  "  veins "  of 
entomologists,  and  the  finer  branches  the  so-called 
nervures. 

These  wings,  when  expanded,  beat  the  air  by 
being  moved  forwards  and  backwards  on  their  point 
of  articulation  to  the  thorax ;  this  is  effected  by  special 
flexor  and  extensor  muscles,  the  number  of  which  is 
considerable,  and  each  of  which  consists  only  of  a  few 
fibres;  in  considering,  however,  the  causes  which  give 
their  particular  direction  to  the  wings  as  they  move 
in  flight,  due  attention  must  be  given  to  the  effects  of 
the  resistance  of  the  air  which  is  beaten  by  the  wing, 
which,  as  a  matter  of  fact,  follows  a  figure  of  8  course. 
In  studying  the  mechanism  of  the  wing  we  have  to 
bear  in  mind  that  the  essential  points  are  a  rigid 
anterior  nervure,  and  a  flexible  membrane  behind 
(Marey). 

Insects  vary  considerably  in  the  number  of  move- 
ments of  the  wing  per  second,  as  may  be  seen  by  the 
following  table,  which  we  owe  to  Marey  : 

Common  Fly 330 

Drone-fly 240 

Bee 110 

Wasp 130 

Humming-bird  Moth 72 

Dragon-fly 28 

Butterfly  (Pontia  rapes)         ....  9 

Among  the  Apterous  insects,  or  those  Hexapods 
in  which  wings  have  never  been  developed,  and  which 
must  be  carefully  distinguished  from  those  that  have, 
owing  to  parasitic  habits  and  so  on,  lost  wings,  which 
were  possessed  by  their  ancestors,  the  Collembola  or 
Spring-tails  are  remarkable  for  the  possession  of  a 
fork-like  appendage  to  the  hinder  end  of  their  abdomen, 
which  can  be  bent  backwards,  and  act  like  a  spring. 


380  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

In  the  Mollusca  the  characteristic  organ  of  loco- 
motion is  the  foot,  which  is  made  up  of  muscular 
fibres,  which  are  transversely  striated,  but  are 
spindle-shaped  cells,  and  so  have  the  general  form  of 
unstriated  muscular  tissue.  This  foot,  which  can  be 
withdrawn  into  the  shell  of  such  forms  as  Anodon  by 
retractor  muscles  attached  anteriorly  and  posteriorly, 
seems  to  be  protruded  or  put  into  a  state  of  erection 
by  an  increased  flow  of  blood  into  its  substance,  and 
not,  as  has  sometimes  been  supposed,  by  the  intaking 
of  water  from  without.  While  it  has  a  somewhat 
conical  or  hatchet- shaped  form  in  the  fresh-water 
mussel,  and  in  those  Lamellibranchs  which  move  about 
with  some  activity,  it  is  very  strong  in  boring  forms 
such  as  Solen,  and  long  and  curved  in  Trigonia,  where 
it  is  used  as  a  leaping  organ  ;  on  the  other  hand,  it  is 
very  small  in  the  scallop  (Pecten),  and  quite  incon- 
spicuous in  the  still  more  sedentary  oyster. 

Among  the  Gastropoda  the  foot  has  often,  as  in 
the  common  snail,  a  broad  disc-like  lower  surface,  and 
is  adapted  for  creeping  or  crawling.  When  the  snail 
is  in  movement  waves  of  contraction  may  be  seen 
passing  over  the  lower  surface  of  the  foot  from  behind 
forwards,  and  it  has  been  found  that  smaller  have 
greater  locomotor  power  than  larger  forms.  Within 
limits,  snails  are  able  to  carry  weights,  and  it  follows, 
therefore,  that  unloaded  snails  do  not  make  use  of  all 
the  activity  of  which  they  are  capable. 

The  foot  may  become  modified  in  a  most  remark- 
able manner,  as,  for  example,  in  the  Heteropoda, 
which  are  forms  found  only  on  the  surface  of  the 
ocean ;  the  animal  swims  with  its  shell  downwards, 
and  its  foot  (Fig.  161 ;  f)  is  converted  into  a  high 
crest-like  fin,  which  is  no  doubt  aided  functionally  by 
the  fin-like  prolongation  of  the  hinder  end  of  the 
body. 

In  the  Pteropoda  the  sides  of  the  foot  become 


chap,  x.]       MOVEMENTS  OF  CHORD  AT  A.  381 

greatly  enlarged,  and  form  distinct  epipodia,  and 
these,  either  independently  or  in  conjunction  with  the 
median  part  of  the  foot,  become  converted  into 
powerful  muscular  fins.  In  the  Cephalopoda  the 
epipodia  form  a  funnel,  through  which  the  water  of 
respiration  is  expelled  to  the  exterior ;  this  expulsion 
of  the  water  forwards  results  in  a  backward  movement 
of  the  animal.  In  the  Tetrabranchiata  (Nautilus)  the 
edges  of  the  epipodia 
are  not,  as  in  th 
Dibranchiata,  fused 
with  one  another,  but 
merely  folded  over. 

Among  the  Chor- 
clata    locomotion    is 

effected  by  swimming,  rte-  161.-Cfcrroewta  cymlium. 

i  n  m  m*  r>  a-          r»T>PPr>in  o-       m>  Proboscis ;  t,  tentacles :  /,  foot ;  d,  disc  ; 
jumping,  eeping,  s,  shell;  g,  branchiae. 

walking,  or  flying,  and 

all  these  activities  are  presented  by  marine  as  well  as 

by  terrestrial  forms. 

Swimming  organs  have  the  form  of  more  or  less 
broad  plates,  which  may  or  may  not  be  supported  by 
bone.  The  simplest  cases  are  presented  by  the  mere 
flattening  or  expansion  of  an  organ  ;  this,  for  example, 
obtains  in  the  tadpole,  the  newt,  or  the  insectivorous 
form  Potamogale  velox,  where  the  tail  is  flattened 
from  side  to  side  and  forms  a  powerful  locomotor 
organ.  In  the  Cetacea,  on  the  other  hand,  which 
require  to  come  repeatedly  and  rapidly  to  the  surface 
of  the  water,  the  tail  is  flattened  from  above  down- 
wards. 

In  more  complicated  cases,  as  in  many  fishes,  the 
tail,  which  is  here  also  the  most  important  organ  of 
locomotion,  and  has  a  screw-like  movement,  is  aided 
by  the  caudal  fin  when  that  is  well  developed ;  the 
paired  lateral  fins  are  in  most  cases  rather  organs  of 
support  and  direction  than  of  locomotion  ;  but  in  some 


382  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

cases,  as  in  the  Rays,  movement  is  almost  altogether 
effected  by  the  undulation  of  the  margins  of  the  enor- 
mous pectorals.  In  flat-fishes  and  eels  locomotion  is 
due  to  the  undulations  or  curvatures  of  the  whole 
body. 

In  other  aquatic  Vertebrates,  such  as  the  marine 
turtles,  the  penguins,  or  the  whales,  where  the  limbs 
take  some  share  in  their  movements  through  the  water, 


Fig.  162.—  Exoccetus  volitans. 

the  tendency  is  for  the  flipper  to  become  a  more  or 
less  rigid  organ,  movable  only  at  its  point  of  attach- 
ment to  the  body.  The  series  of  modifications  which 
lead  to  this  arrangement  are  very  well  seen  among 
the  Clielonia.  In  the  marsh  tortoises  the  digits  are 
united  by  a  web,  but  each  digit  has  a  claw ;  in  the 
mud  tortoises  the  limbs  are  flatter,  and  there  are  claws 
on  only  three  of  the  digits,  while  in  marine  turtles 
the  still  more  flattened  digits  are  united  by  a  common 
covering  of  skin  into  a  more  rigid  paddle,  and  only 
one  or  two  claws  are  found.  In  the  penguin  the 
wings  are  converted  into  firm  paddles,  movable  only 


Chap,  x.]  FLYING  ORGANS.  383 

at  their  base.  In  the  Cetacea  all  the  bones  of  the 
fore  limb  are  united  in  a  common  integument,  and 
form  the  "  flipper." 

Some  forms  escape  with  rapidity  by  making 
bounds  or  jumps ;  of  these  we  have  examples  in  the 
frog,  the  kangaroo,  or  the  Cape  jumping-hare,  in  all  of 
which  the  hind  limbs  are  much  stronger  and  longer 
than  the  fore  limbs. 

Creeping  or  crawling  is  best  seen  in  the  snakes, 
which  move  along  the  ground  by  the  backward  and 
forward  movement  of  their  ribs,  which  they  use  as 
stilts. 

Flying  organs  are  found  among  fishes  in  Exoccetus, 
where  the  pectoral  fins  are  so  greatly  elongated  as  in 
some  species  to  reach  as  far  back  as  the  caudal  fin ; 
the  fins  are  not  actively  moved,  and  seem  to  have  no 
power  of  turning  the  fish  to  the  right  hand  or  the 
left ;  they  cannot  fly  far  at  a  time. 

Similarly  modified  pectorals  are  found  in  Dactyl- 
op  terus. 

Among  recent  Reptiles,  Draco  volans,  the  dragon, 
or  flying  lizard,  is  capable  of  short  movements  through 
the  air,  owing  to  the  prolongation  of  some  of  its  ribs, 
which,  when  covered  with  the  skin,  form  a  semi- 
circular wing  on  either  side  of  the  body.  The  extinct 
Pterosaurians  (Pterodactyle)  had  the  outer  digit  of 
the  manus  as  long  or  longer  than  the  rest  of  the  fore 
limb ;  and  there  is  evidence  that,  as  in  the  bats,  the 
integument  was  produced  on  either  side  into  a  mem- 
brane, the  outer  edge  of  which  was  attached  to  this 
digit,  and  so  formed  an  expansion,  by  means  of  which 
the  creature  was  enabled  to  support  itself  in  the  air. 

Among  Mammals  the  organs  of  flight  are  best 
developed  in  the  Chiroptera  (bats)  (Fig.  163),  where 
they  are  formed  by  the  modification  of  the  skeleton, 
and  especially  of  the  fore-arm  (see  Fig.  149),  and  by 
the  extension  of  the  integument  into  the  so-called 


384  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Chap,  x.]      FLYING  ORGANS  OF  MAMMALS.  385 

volar  membranes.  These,  when  best  developed,  con- 
sist of  (1)  an  "  antebrachial  membrane,"  which  extends 
from  the  shoulder  to  the  base  of  the  thumb ;  (2)  the 
"  wing  membrane,"  which  extends  from  the  sides  of 
the  body  along  the  fore-arm  and  between  the  elongated 
digits  of  the  manus  ;  (3)  an  "  interfemoral  membrane," 
which  is  attached  to  the  hinder  end  of  the  body,  and 
to  the  sides  of  the  leg  as  far  as  the  heel,  and  in  some 
as  far  as  the  phalanges  of  the  foot.  The  relation  of 
the  antebrachial  membrane  to  the  power  of  flight  is 
spoken  to  not  only  by  the  extent  of  its  development 
in  flying  forms,  but  also  by  its  reduction  in  such  as 
are  best  fitted  for  terrestrial  progression.  The  most 
important  function  of  the  interfemoral  membrane 
would  appear  to  be  that  of  acting  as  a  rudder ;  this 
power  is  greatest  when,  as  in  the  Molossi,  the  bat  is 
able  to  vary  the  extent  of  the  membrane,  for  this 
"must  confer  upon  them  great  dexterity  in  quickly 
changing  the  direction  of  their  flight,  as  when  obliged 
to  double  in  pursuing  their  swiftly-flying  insect  prey  " 
(Dolson). 

Less  well-marked  powers  of  flight  are  possessed  by 
the  aberrant  insectivore  Galeopithecus  (the  so-called 
"  flying  lemur  "),  which  has  been  observed  to  move 
through  seventy  yards  of  air,  and  in  which  the  two 
pairs  of  limbs  and  the  tail  are  connected  together  by 
an  expansion  of  the  skin,  which  forms  a  parachute- 
like  enlargement ;  this  is  not,  however,  merely  mem- 
branous, as  in  bats,  but  is  hairy  on  either  surface. 
Among  the  Rodents  the  flying  squirrel  (Pteromys), 
and  among  Marsupials  the  flying  phalanger  (Petau- 
rus),  have  the  fore  and  hind  limbs  connected  by  a  fold 
of  skin,  which,  when  the  limbs  are  extended,  forms  a 
similar  kind  of  parachute,  but  it  does  not  reach  to  the 
tail,  nor  is  their  patagium  provided,  like  that  of  Galeo- 
pithecus, with  any  special  muscles. 

Organs  of  flight  are,  among  the  Yertebrata,  best 
z— 16 


386    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


developed,  as  we  know,  in  certain  Birds  •  the  skeleton 
of  the  fore-arm  is  specially  modified  (see  page  352),  and 
forms  for  the  wing  a  firm  anterior  bar,  comparable  to 
the  anterior  nervure  of  the  insect's  wing ;  this  bar  is 

moved  by  special  mus- 
cles, which  are  at- 
tached near  its  base  ; 
but  all  of  which  lie 
on  the  ventral  or 
lower  surface  of  the 
body,  and  thereby 
enable  the  centre  of 
gravity  of  the  bird 
to  be  lower  than  it 
would  be  were  the 
extensor  muscles  of 
the  arm  placed,  as  in 
other  Vertebrates,  on 
the  dorsal  surface.  A 
large  surface  of  at- 
tachment for  the  pec- 
toral muscles  is  ob- 
tained by  the  great 
development  in  flying 
birds  of  the  keel  of 
the  sternum  (see  page 
347),  and  the  extensor 
muscle  works  on  a 
pulley.  The  greater 
portion  of  the  wing  is 

not  formed  by  membrane  or  integument,  but  by  the 
development  of  those  integumentary  structures  which 
we  call  feathers.  These  feathers  overlap  one  another 
in  such  a  way  that  the  wing  is  convex  above  and 
concave  below,  and  that  pressure  from  below  forces 
the  feathers  more  closely  together.  From  this  arrange- 
ment it  is  clear  that  in  the  up  and  down  movement  of 
the  wing  in  the  air,  much  greater  effect  is  gained  by 


Fig.  164.— The  Common  Swift. 


Chap.  XL]  VOCAL  ORGANS.  387 

the  down-stroke  than  by  the  up-stroke ;  for,  in  the 
first  place,  the  pressure  of  air  on  a  concave  surface  is 
always  more  effectual  than  that  on  a  convex  ;  an 
umbrella,  for  example,  may  be  blown  inside  out,  but 
never  outside  in  ;  in  the  next  place,  the  pressure  of 
the  air  against  the  separate  feathers  welds  them  into 
a  connected  whole,  while  in  the  up-stroke  the  air  not 
only  meets  with  a  convex  surface  from  which  it  may 
flow  away,  but  it  is  also  able  to  escape  between  the 
separate  feathers.  The  influence,  therefore,  of  gravity 
is  overcome  by  the  greater  value  of  the  down-stroke, 
and  by  the  diminution  of  the  pressure  of  air  in  the 
less  valuable  up-stroke,  which  can  be  made  more 
rapidly  than  the  down-stroke.  A  further  inquiry 
into  the  complicated  question  of  the  mechanism  of 
flight  would  lead  us  beyond  the  scope  of  this  work. 
In  some  cases  the  tail  feathers,  by  being  raised,  de- 
pressed, or  turned  a  little  to  one  side,  are  able  to  give 
an  upward,  downward,  or  oblique  course  to  the  bird. 


CHAPTER    XL 

VOCAL  ORGANS. 

UNDER  the  head  of  vocal  organs  we  may  group  all 
those  which  produce  distinct  and  definite  sounds  to 
the  human  ear,  or  which  may  be  supposed  to  similarly 
affect  the  auditory  nerves  of  other  animals. 

These  organs  are  never  developed  among  the  lower 
Metazoa ;  indeed,  so  far  as  we  know  at  present,  they 
are  confined  to  the  Arthropoda  and  the  Vertebrata. 
Among  the  Crustacea  they  have  been  detected  in 
Palinurus. 

In  several  orders  of  Insects  they  are  confined  to 
the  male  sex,  and  appear,  therefore,  to  be  means,  as 


388  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


they  are  also  no  doubt  in  birds,  by  which  the  male 
may  attract  the  female. 

They  are  so  commonly  developed  in  the  Ortliop- 
tera  (grasshoppers  and  crickets),  that  the  arrange- 
ment which  obtains  in  one  member  of  this  order  may 
be  conveniently  taken  as  a  type.  In  Macrolyristes 

imperator  (Fig. 
165 ;  A  and  B) 
we  observe  that 
the  hinder  bor- 
der of  the  right 
wing  (s)  is  thick- 
ened in  such  a 
way  as  to  act  as 
a  cord,  and  that 
another  part  of 
the  wing  (m)  is 
converted  into  a 
tense  membrane. 
The  left  wing  (B) 
has  its  lower 
surface  rough- 
ened like  a  file 
along  one  line  ; 
this  file  is 
brought  to  rub 
upon  the  thick 
cord  (s)  of  the 
right  wing,  and 

so  sets  the  membrane  (m)  in  vibration  ;  vibrations  are, 
of  course,  conveyed  to  the  air,  and,  being  regular  and 
definite,  they  set  up  vibrations  in  the  air  which,  on  strik- 
ing the  auditory  nerve,  give  rise  to  the  sensation  of  more 
or  less  musical  sounds.  Somewhat  similar  structures 
are  to  be  found  on  the  wings  of  the  locust,  and  in  the 
field  cricket ;  in  the  latter  the  two  wings  are  similar 
in  structure,  and  their  movement  on  one  another  can, 


Fig.  165.— The  Sound-producing  Organ  of  the 
Orthopterous  insect  Macrolyristes  imperalor. 

A,  Upper  view  of  right  wing  ;  s,  cord  ;  m,  membrane ; 
B,  lower  view  of  left  wing ;  b,  roughened  edge. 


Chap,  xi.]          VOCAL  ORGANS  OF  INSECTS.  389 

therefore,  be  reversed.  In  the  grasshoppers  the  sound- 
producing  organs  are  developed  not  on  the  wings  but 
on  the  legs,  the  upper  joints  of  which  are  provided 
with  rather  less  than  one  hundred  minute  denticles 
which  scrape  on  the  wings ;  in  the  males  of  an  allied 
form  (Pneumora),  the  legs  are  rubbed  against  a  notched 
ridge  which  is  developed  on  either  side  of  the  abdo- 
men, and  the  resonance  is  greatly  increased  by  the 
whole  body  being  distended  with  air.  In  most  cases 
among  the  Orthoptera  the  males  are  alone  vocal,  and 
the  object  of  the  use  of  these  organs  is,  no  doubt,  that 
of  attracting  the  female. 

In  the  hemipterous  Homoptera,  of  which  the  Cicadas 
are  members,  and  of  which  the  males  are  alone  vocal, 
the  sound  seems  to  be  produced  by  the  vibration  of 
membranes,  placed  on  either  side  of  the  stigmata  of 
the  metathorax,  and  set  in  motion  by  the  respiratory 
air. 

The  Hymenoptera,  among  which  are  the  bees  that 
hum,  would  appear  to  produce  sounds  by  the  move- 
ment of  the  abdominal  segments  on  one  another ; 
these,  as  Mr.  Darwin  has  observed,  are  marked  with 
very  fine  concentric  ridges,  such  as  are  found  also  on 
the  thoracic  collar,  with  which  the  head  articulates. 
Among  the  Coleoptera  (beetles),  there  are  forms  such 
as  the  carrion  beetles  (Necrophorus),  and  others 
which  make  very  distinct  sounds ;  these  are  ordinarily 
produced  by  rasped  ridges,  which  are  placed  on  various 
parts  of  the  body  and  worked  against  the  edges  of  the 
elytra  or  wing-covers ;  or  parts  of  the  leg  work  against 
ridges  on  the  abdomen ;  or  the  elytra  are  ridged,  as  in 
some  of  the  water  beetles ;  or,  lastly,  two  of  the  seg- 
ments of  the  thorax  may  work  on  one  another ;  in  the 
latter  case  the  ridges  may  be  developed  either  on  the 
upper  or  on  the  lower  surface.  The  vocal  or  stridu- 
lating  organs  of  Coleoptera  appear  to  be  equally  or 
nearly  equally  developed  in  both  sexes,  and  it  is  rare 


390    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

for  the  male  to  be  much  better  provided  with  them 
than  the  female. 

Sound-producing  organs  are  much  less  common 
among  butterflies  and  moths,  and  where  present, 
they  seem  to  be  due  to  the  vibration  of  a  membrane, 
and  not  to  the  movement  of  a  rasping  organ,  as  in 
beetles. 

Among  the  Yertebrata,  voice,  or  definite  and  more 
or  less  musical  sounds,  are  ordinarily  produced  by  the 
vibration  of  the  column  of  air  which  passes  down  the 
trachea  and  sets  in  movement  the  membranes  (vocal 
membranes),  which  lie  on  either  side  of  that  por- 
tion of  the  trachea  which  is  distinguished  as  the 
larynx  ;  they  are  supported  by  definite  cartilaginous 
pieces  (arytenoid  cartilages),  and  bound  a  narrow 
cleft  which  is  known  as  the  glottis.  While  this 
simple  condition  is,  for  example,  found,  among  the 
Amphibia,  in  some  frogs,  others  have  well-developed 
sacs  connected  with  the  larynx,  which  become  swollen 
out  and  project  on  either  side  of  the  head  ;  these  sacs, 
which  are  often  better  developed  in  males  than  females, 
take  an  important  share  in  increasing  the  noise  made 
by  their  possessor,  which  may  sometimes  be  heard  at 
a  great  distance. 

Among  Reptiles,  where  the  laryngeal  apparatus  is, 
on  the  whole,  comparatively  simple,  the  chameleons 
are  provided  with  air  sacs,  which  do  not  appear  011 
the  surface  of  the  animal  as  they  do  in  the  edible 
and  some  other,  though  not  all,  frogs. 

Birds  are  remarkable  for  the  fact  that  their  vocal 
organ  is  not,  as  in  other  Vertebrates,  formed  at  the 
top,  but  at  the  bottom  of  their  trachea,  and  at  the 
point  where  the  trachea  divides  into  the  two  bronchi  ; 
the  syrinx,  as  the  organ  of  voice  in  birds  is  called,  is 
best  developed  in  the  Passeres,  where  a  share  in  its 
formation  is  taken  both  by  the  trachea  and  by  the 
bronchi  (broncliio-traclieal  syrinx). 


Chap.  XI.] 


VOCAL  ORGANS  OF  BIRDS. 


391 


Some  of  the  lower  rings  of  the  trachea  unite  to  form 
a  tympanic  chamber ;  the  tracheal  orifices  of  the  two 
bronchi  are  separated  by  a  membranous  septum,  and 
on  either  side  there  is  a  tympaniform  membrane 
formed  on  the  inner  side  of  the  uppermost  bronchial 
rings  ;  the  air  which  passes  through  the  bronchial  clefts 
sets  in  vibration  the  membranes  which  bound  them,  and 
the  character  of  the  note  produced  is  affected,  on 
purely  physical  principles, 
by  the  position  of  the 
bronchial  half-rings,  and 
by  the  length  of  the 
column  of  air  in  the 
trachea.  The  position  of 
these  half-rings  is  not 
constant,  owing  to  the 
fact  that  they  are  moved 
by  proper  muscles,  which 
act  on  their  ends,  and  so 
rotate  them. 

In  Steatoriiis  (one  of 
the  night-jars),  the  syrinx 
is  completely  bronchial, 
the  fifteenth  and  sixteenth 
bronchial  rings  being  only 

half-rings,  as  are  also  some  that  follow  them ;  the 
space  thus  formed  is  filled  in  with  membrane,  which 
can  be  rendered  tenser  by  the  contraction  of  the 
lateral  muscle  of  the  trachea,  which  is  attached  to  the 
middle  of  the  fifteenth  ring.  In  those  American 
crows  in  which  the  syrinx  is  completely  tracheal,  we 
have  an  arrangement  which  is  essentially  similar. 
Among  the  Katite  birds  the  syrinx  is  best  developed 
in  Rhea ;  in  the  American  vultures  the  voice  organ  is 
found  in  its  simplest  condition. 

It  is  obvious  that  the  length  of  the  trachea  must 
have  a  very  considerable  influence  011  the  character  of 


Fig.  166. — Larynx  of  Peregrine 
Falcon.  A,  Front  view;  B,  in 
section. 


392  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  note  or  notes  emitted  by  a  bird  ;  but  as  yet  we 
have  no  definite  information  as  to  the  meaning  of 
those  convolutions  of  the  trachea  which  are  so  com- 
mon in  swans  and  ducks,  and  sometimes  give  to 
the  tube  the  appearance  of  a  French  horn  (Darwin). 
In  some  grouse  resonance  is  aided  by  the  development 
of  air  sacs  which  are  capable  of  inflation ;  the  great 
throat-pouch  of  the  European  bustard  appears  to 
have  a  similar  function. 

Among  the  Mammalia  the  larynx  becomes  re- 
markably complicated,  a  number  of  special  cartilages 
being  developed,  which  are  connected  together  by 
ligaments,  and  moved  on  one  another  by  special  mus- 
cles ;  the  whole  function  of  this  apparatus  is  to  alter 
the  form  of  the  slit  of  the  glottis,  and  to  increase  or 
diminish  the  tension  of  the  vocal  cords.  As  this  sub- 
ject has  been  already  dealt  with  in  chap.  xv.  of  the 
"  Elements  of  Human  Physiology,"  we  have  here  only 
to  point  out  that  Mammals  differ  greatly  in  the  sounds 
that  they  make,  the  dog  barking,  the  cat  mewing,  the 
lion  roaring,  but  that  most  agree  in  using  the  voice 
more  at  the  breeding  season  than  at  any  other  ;  a  few 
mammals,  such  as  the  American  Hesperomys  cornutus, 
and  the  gibbon  (Hylobates),  which,  it  is  interesting 
to  observe,  is  one  of  the  anthropoid  or  man-like  apes, 
may  be  distinctly  said  to  sing.  Man  is  remarkable 
for  his  capacity  for  producing  not  only  sounds,  but 
articulate  speech,  the  wealth  and  extent  of  which  is 
much  greater  in  the  higher  than  in  the  lower  races  of 
his  species. 

In  Fishes,  sounds,  when  produced,  are  of  course 
but  rarely  associated  with  the  passage  into  the  air 
bladder ;  but  Ceratodus  has  been  observed  to  make 
grunting  noises,  which  are  possibly  involuntary. 
Various  Cyprinoid  and  Siluroid  fishes  are  known  to 
make  sounds,  and  in  Callomystax,  Haddon  has  dis- 
covered that  the  agent  by  which  they  are  produced 


Chap,  xii.]  THE  NERVOUS  SYSTEM.  393 

are  the  anterior  neural  spines ;  these  rub  on  the  suc- 
ceeding and  more  solid  portion  of  the  vertebral 
column. 


CHAPTER  XII. 

THE    NERVOUS    SYSTEM    AND    ORGANS    OF    SENSE. 

THE  nervous  system  of  an  animal  is  the  apparatus 
by  means  of  which  it  becomes  acquainted  with  what 
is  going  on  around  it,  is  enabled  to  distribute  that 
information  throughout  itself,  or  to  bring  it  to  some 
central  region,  and  to  set  itself  in  proper  relation  to 
the  surrounding  medium.  In  consequence  of  this 
relation  to  the  outer  world,  we  -find  that  the  system 
is,  at  first,  superficial  in  position  and  diffused  in 
arrangement,  that  is  to  say,  it  at  first  lies  in  the  outer 
layer  of  the  body,  with  which,  indeed,  it  at  all  times 
remains  closely  connected ;  and  that,  primitively,  the 
system  is  more  or  less  equally  distributed  throughout 
the  whole  of  the  organism. 

As  we  know,  the  Protozoa  have  no  definite 
nervous  system,  but  we  have  already  learnt  than  an 
Amoeba  is  so  far  sensitive  that  stimuli  applied  to  its 
surface  are  followed  by  changes  in  the  disposition  of 
its  protoplasm.  Nor  have  we  any  knowledge  of  a 
nervous  system  in  Sponges.  (See  page  431.) 

In  all  other  groups  of  the  Metazoa  we  have 
evidence  of  the  presence  of  cells  set  apart  for  the 
genera]  function  of  informing  the  organism  of  what 
is  going  on  around  it. 

When  we  inquire  as  to  what  are  the  essential  con- 
stituents of  a  nervous  system,  we  find  that  they  are 
either  central  (ganglionic)  cells  or  conducting 
fibres ;  and,  as  we  advance  through  the  scale  of 
organisation,  we  observe  that  both  cells  and  fibres 


394  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

undergo  aggregation,  so  that  a  diffused  or  scattered 
arrangement  makes  way  for  one  in  which  we  have 
definite  nerve  centres  and  well-marked  lines,  along 
which,  and  along  which  only,  nervous  impulses  will 
pass.  The  most  important  of  the  aggregations  of 
nerve  cells  form  the  brain,  or  cerebrum,  the  most 
important  of  the  fibres  the  nerve  cords;  and  just 
as  nerve  fibres  going  to  or  coming  from  the  latter  are 
associated  with  them,  so  there  are  secondary  masses 
of  ganglia  which  are  connected  with  the  former. 

In  the  next  place,  information  from  without  is 
gained  from  specially-modified  cells,  sense  cells ; 
these  belong  to  the  epithelial  region  of  the  body,  and 
are  derivates  of  the  epiblast. 

The  most  generalised  and  widely-distributed  sense 
cells  are  those  which  belong  to  the  sense  of  touch,  the 
so-called  tactile  cells;  next  we  have  those  which, 
only  a  little  more  complex,  are  confined  to  the  ante- 
rior region  of  the  digestive  tract ;  these  are  the 
gustatory  cells,  or  those  that  subserve  the  sense  of 
taste  ;  thirdly,  we  have  the  more  complicated  organs 
of  the  three  higher  senses,  smell,  sight,  and 
hearing^.  When  a  brain  is  developed,  all,  or  such 
of  these  organs  as  are  present,  send  to  it  by  the 
nerve  fibres  messages  from  the  outer  world  ;  in  it 
the  messages  are  converted  into  more  or  less  distinct 
sensations,  and  from  it  fresh  messages  are  sent  out  to 
the  different  parts  of  the  body. 

The  relations  of  the  sensory  cells  to  the  epithelial 
layer  are  particularly  well  seen  in  some  of  the 
Coelenterata ;  for  example,  in  the  sea-anemones 
(Tealia;  Fig.  168),  some  of  the  cells  of  the  epithelial 
layer  have  their  free  end  continued  into  a  fine 
stiff"  process,  which  projects  outwards  ;  the  inner  or 
basal  end  of  such  cells  breaks  up  into  finer  pro- 
cesses, which  branch  towards  their  ends.  The  free 
projecting  process  may  be  justly  regarded  as  a 


Chap,  xii.]  NERVOUS  SYSTEM  OF  CCELENTERATA.  395 

sense  hair,  which,  acted  on  by  movements  in  the 
water,  and  communicating  with  the  body  of  the  cell, 
is  able  to  bring  the  animal  into  relation  with  the 
outer  world. 

In  the  sea-anemones  the  basal  processes  of  the  cells 
have  been  observed  to  be  continued  into  a  layer  of 


Fig.  167.— Part  of  the  submuscular  pleius  of  Amelia  aurita,  showing 
ganglionic  cells.    (After  Schiifer.) 


fibres,  which  are,  to  all  appearance,  nervous  in  nature. 
Well-developed  ganglionic  cells  are  to  be  found 
scattered  in  the  layer  of  nervous  fibres  which  sur- 
rounds the  mouth. 

While  in  Aurelia  and  other  Acraspedote  Me- 
dusae the  central  part  of  the  nervous  system  consists 
of  isolated  ganglia,  ordinarily  eight  in  number,  the 
Craspedote  Medusse,  or  those  in  which  the  edge 
of  the  bell  is  provided  with  a  velum,  have  a  more 
definite  central  system;  the  epithelial  coverings  of 


396    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


both  the  upper  and  lower  surfaces  of  the  velum  have 
some  of  the  constituent  cells  converted  into  sense 
cells  ;  the  basal  ends  of  these  are  of  some  length,  and 
pass  into  a  nervous  ring  which  runs  round  the  edge 
of  the  bell.  The  several  sensory  cells  are  thereby 

brought  into  connection 
with  one  another,  and  the 
consentaneous  action  of 
all  parts  of  the  jelly-fish 
is  thus  ensured. 

Underlying  the  epi- 
thelium of  the  lower  sur- 
face of  the  bell,  and  placed 
between  it  and  the  mus- 
cles, is  a  network  of  nerve 
fibres,  among  which  there 
are  scattered  ganglionic 
cells  (Fig.  167);  this  net- 
work is  connected  with 
the  marginal  nerve-ring. 
Here,  then,  we  have  a 
simple  example  of  an 
aggregated  central  ner- 
vous system,  together 
with  a  peripheral 
system  of  fibres  and  cells, 
which  is  diffused  over  the  whole  of  the  under  surface 
of  the  bell  of  the  medusa.  Some  of  the  Craspedota 
(e.g.  Carmarina)  present  us  with  an  important  advance 
in  structural  differentiation,  for  some  of  what,  in  all 
other  particulars,  resemble  the  sense  cells,  are  found  to 
have  lost  their  free  projecting  process,  and  to  be  now 
moved  a  little  away  from  the  surface  of  the  body. 
Here,  then,  we  have  nervous  epithelial  cells  which 
are  beginning  to  lose  their  superficial  position,  and 
sinking  deeper  into  the  substance  of  the  or- 
ganism.. 


Fig.  168. — Transverse  Section 
through  aTentacle  of  Tealia  cras- 
sicornis ;  to  show  (a)  Sensory 
Cells  with  their  free  Projecting 
Processes,  and  their  Bases  con- 
tinued into  the  Nervous  Layer; 
(b),  supporting  cells. 


Chap.  XII.] 


MEDUSM. 


397 


Experiments  on  Medusae  show  that  the  seat  of 
spontaneous  activity  is  confined  to  the  edge  of  the 
bell  in  the  Craspedote  Me- 
dusse,  and  to  the  region  of 
the  marginal  sense  organs 
in  the  Acraspedote  forms; 
if  the  extreme  margin  of  the 
bell  of  the  former  be  com- 
pletely removed,  there  is 
immediately  a  total  and 
permanent  paralysis  of  the 
entire  organ  ;  in  the  latter, 
removal  of  the  marginal 
bodies  is  sufficient  to  pro- 
duce a  similar  effect.  The 
results  of  these  experiments 
are,  then,  in 
complete  ac- 
cordance with 
the  anatomical 
facts.  The  dif- 
fused plexiform 
arrangement  of 
the  nerve  fibres 
is,  further,  spo- 
ken to  by  the 
following  ex- 
periment :  if  all 
the  marginal 
sense  organs  but 
one  be  removed, 
and  if  deep 
sections  be  made  in  the  substance  of  the  bell, 
so  as  to,  at  any  rate,  separate  the  nerve  fibres  at 
many  points  of  their  course,  it  is  found  that  the 
bell  is  still  capable  of  contraction  ;  or,  in  other 
words,  the  stimuli  sent  out  from  the  sole  remaining 


398  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

centre  are  able  to  diffuse  themselves  over  the  whole 
substance  of  the  jelly-fish. 

We  have,  it  is  clear,  to  consider  the  nervous  system 
as  at  first  forming  a  diffused  network  over  the 
whole  body,  and  this  truth  must  be  constantly  borne 
in  mind,  for  it  applies  not  only  to  the  Ccelenterata, 
but  also  to  the  lowest  worms.  At  the  same  time, 


Fig.  170.— Diagrams  to  show  the  relative  positions  of  the  longitudinal 
Nerve  Cords  in  different  genera  of  Nemerteans.  The  epidermal 
tissues  are  left  white,  the  muscles  are  darker,  and  the  nerve  cords 
are  darker  still.  A,  Carinella ;  B,  Cerebr.itulus  ;  c,  Langia  ;  D,  Amphi- 
porus;  E,  Drepanophorus.  (After  Hubrecht.) 

we  have  to  note  the  tendency  of  the  nerve  cells  and 
fibres  to  seek  a  more  sheltered  position  than  that 
which  can  be  afforded  them  by  the  surface  of  the 
body ;  nowhere,  perhaps,  are  the  various  stages  of 
modification  better  seen  than  among  the  TVemertean 
worms,  of  which  Carinella  is  one  of  the  lowest  and 
simplest  examples. 

It  will  be  seen  that  in  the  figure  (Fig.  170)  Cari- 
nella (A)  has  the  longitudinal  nerve  cords  just  under- 
lying the  epidermal,  and  placed  above  the  muscular 
tissues. 


Chap,  xii.]    NERVOUS  SYSTEM  OF  NEMERTINES.      399 

In  Cerebratulus  and  Langia  (B,  c)  they  lie  in  the 
midst  of  the  muscular  tissue ;  while  in  Amphiporus 
and  Drepanophorus  they  are  internal  to  it,  as  they 
are  in  the  greater  number  of  invertebrate  Metazoa. 


Fig.  171.— Outer  surface  of  a  decalcified  Plate  of  the  Test  of  Brissopsis 
lyrifera,  from  the  greater  part  of  which  tbe  connective  tissue  (ct) 
has  been  removed,  to  show  the  course  of  the  Peripheral  Nerve- 
fibres,  and  their  ganglionic  cells.  Highly  magnified.  (After  Loven.) 


Carinella  is,  moreover,  remarkable  for  the  fact 
that  the  centralisation  of  ganglia  and  nerve  cords  has 
proceeded  to  a  small  .degree  only.  As  in  all  Nemer- 
tines,  the  ganglia  are  distributed  over  the  whole 
course  of  the  longitudinal  nerve  trunks,  and  what,  in 
other  forms,  is  an  anterior  cerebral  enlargement,  is  here 
merely  represented  by  the  enlargement  of  the  front 


400  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

end  of  the  lateral  trunks.  Connected,  finally,  with 
the  two  chief  nerve  trunks  is  a  network  of  nervous 
cells  and  fibres,  which  lies  just  below  the  dermis,  and 
forms  a  continuous  layer  over  the  whole  of  the  worm. 

In  the  Turfoellaria  we  find  also  that  the  nervous 
system  is  superficial  in  position,  and  that  the  nerve 
fibres  so  branch  as  to  be  distributed  widely  over  the 
surface  of  the  body. 

A  similarly  primitive  condition  obtains  in  the 
I :<  h i iKHh'iiii:! I :i  •  the  epidermis  consists  not  only  of 
supporting:  cells,  but  of  others  which  are  sensory? 
and  have  their  basal  ends  continued  into  nerve  fibrils, 
which  ordinarily  run  parallel  to  the  surface  of  the 
body ;  with  these  fibrils  small  ganglion  cells  are  con- 
nected (Hamann)  ;  as  a  result  of  this,  we  have  a 
continuous  sheath  of  nerve  tissue  investing  the  body 
of  a  starfish  or  of  an  Echinoid  (Fig.  171).  In  the 
Ophiuroid  and  the  Holothurian,  the  superficial  nerve 
plexuses  have  as  yet  been  detected  only  on  the  tube 
feet.  By  far  the  greater  part  of  the  nervous  system 
is  superficial  in  the  starfish,  for  the  nervous  band 
that  runs  down  the  groove  of  every  arm  is  placed  just 
below  the  investing  epithelium  ;  and,  in  addition  to 
this,  the  more  primitive  histological  condition  is  still 
retained,  for  the  ganglia  are  scattered  among  the  nerve 
fibres,  and  not  collected  into  separate  masses. 

Having  now  sufficient  evidence  of  the  truth  of  the 
statement  that  the  nervous  system  is  primitively 
superficial  in  position ;  that  is  to  say,  that  at  first  the 
nerve  cells  lie  side  by  side  with  the  epithelial  cells,  and 
that  they  gradually  come  to  lie  just  below  the  epithe- 
lial layer,  we  may  return  to  that  plexiform  disposition 
of  fibres  which  precedes  the  arrangement  in  definite 
strands  or  cords.  Evidence  as  to  this  is  afforded  by 
the  most  primitive  members  now  existing,  both  of 
the  Arthropoda  and  of  the  Mollusca.  Of  the 
former,  Peripatus  is  a  striking  example  (Fig.  172). 


chap,  xn.j     NER  vo  us  S\  '.v  TEM  OF  PERI  PA  rus.         40 1 


A  .'  aln 


The  ventral  nerve  cords  are  widely  separated  from  one 
another,  but  are 
connected  together 
by  a  large  number 
of  commissures  (co1), 
of  which  there  are 
from  nine  to  ten  for 
each  segment  of  the 
body.  From  the 
outer  borders  of  the 
cords  nerve  fibres  are 
given  off  to  all  parts 
of  the  body,  the 
whole  of  which  is 
consequently  sur- 
rounded by  the  ner- 
vous system ;  and  we 
have  here,  therefore, 
what  is  essentially  a 
plexiform  arrange- 
ment, but  one  which 
has,  so  to  speak,  be- 
come regulated.  A 
further  advance  is  to 
be  found  in  the  fact, 
that  while  the  cords 
are  everywhere  co- 
vered by  ganglion  . 
cells  on  their  ventral 
surface,  the  ganglia 
are  more  especially 
numerous  at  one 
point  in  every  seg- 
ment of  the  body, 

\\  here  they  form  such  an  enlargement  as  that  marked 
fy]  in  Fig.  172. 

Proneomenia  may  be  taken  as  the  simplest  type  of 

A  A--16 


Pig.  172.— Anterior  portion  of  the  central 
Nervous  System  of  Peripaius,  show- 
ing the  Anterior  Cerebral  Ganglia, 
with  the  Lateral  Nerve  Cords  con- 
nected with  one  another  by  numerous 
commissures  (co).  (After  Balfour;) 

E,  Eye  ;  atn,  antennary  nerve  ;  co  l,  first  com- 
missure :  orn,  nerves  for  the  mnutb  ;  ore/, 
oral  ganglion  ;  pn,  pedal  nerves;  fc/', 
first  ganglionic  eulai  gement  for  the 
pedal  nerves. 


APGr 


402  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


the  Mollusca,  but  it  is  impor- 
tant to  note  that  along  this 
phylum  we  have  persisting  a 
larger  number  of  conditions 
than  are  at  present,  at  any 
rate,  known  among  the  Ar- 
thropoda.  A  reference  to  Fig. 
ITS  will  show  that,  in  Pro- 
neornenia,  there  are,  on  either 
side,  two1  cords  which  run 
down  the  whole  length  of  the 
body,  and  both  of  which  ter- 
minate in  a  ganglionic  swel- 
ling ;  the  two  inner  cords  are 
seen  to  be  Connected  with  one 
another  by  commissural  fibres, 
and  each  of  these  with  the  edge 
of  the  cord  that  lies  outside 
it  j  as  these  latter  give  off 
peripheral  nerves  it  follows 
that  here  again  we  have  a 
plexus  of  nerve  fibres  distri- 
buted through  the  body.  In 
the  case  of  Proneomenia  we 
have  ganglion  cells  not  only 
accompanying  the  nerve  fibres 
throughout  the  whole  of  their 
length, -but  they  are  also,  as 
they  are  in  some  of  the  com- 
missures of  Peripatus,  found 
on  the  commissures  which  con- 
nect these  cords  with  one  an- 
other. Here,  then,  we  have 
yet  another  instance  of  the 
plexiform  disposition  of  nerve 
fibres,  and  the  diffused  condi- 
tion of  ganglionic  cells  in  a 


.TPG 
JVC— 


Fig.  173. —  Diagram  of  the 
Nervous  System  of  Proneo- 
menia. 

CX3,  Cerebral  ganglion  ;  sty,  sub- 
lingual  gansrlla  ;  APG,  PPG,  PVO, 
anterior  pedal,  posterior  p  -dal, 
posterior  lateral  (visceral) 
ganglia  ;  el,  sublingual  •  con- 
nectives ;  Cpc,  cerebropedal 
connectives  ;  pe,  longitudinal 
pedal  nerve  trunks  ;  la,  longi- 
tudinal lateral  nerve  trunks. 
(After  HubrechU 


chap  xii.]  NERVOUS  SYSTEM.  403 

lowly  and  little  differentiated  representative  of  a  large 
group  of  animals,  in  the  higher  members  of  which  con- 
centration is  exceedingly  well  marked.  (See  page  411.) 

With  the  exception,  then,  that  in  Peripatus  and; 
Proneomenia,  the  anterior  end  of  the  nerve  cords  is 
enlarged  into  a  cerebral  mass,  we  should  appear- 
to  be  able  to  see  no  essential  difference  between  them 
and  a  Craspedote  Medusa,  save  in  the  fact  that  the 
Medusa  has  a  complete  nerve  ring.  In  so  far,  how- 
ever, as  there  is  in  both  the  Arthropod  and  Mollusc 
just  named,  a  commissure  at  the  hinder  end  of  the 
body  which  connects  the  right  and  left  cords  with  one 
another,  it  is  clear  that  the  nerve  system,  if  not  a 
ring,  is  at  any  rate  a  closed  system ;  that,  in  other 
words,  it  may  be  compared  to  a  ring  drawn  out  length- 
wise (Balfour).  If  this  comparison  be  a  just  one  we 
are  soon  able  to  explain  the  reason  why  the  anterior 
end  of  a  Nemertine  or  Arthropod  or  a  Mollusc  is  better 
developed  than  the  rest  of  the  nerve  cord,  for  these 
animals  are  all  bilaterally,  in  place  of  being  circularly 
or  radially,  symmetrical ;  and  it  follows,  therefore,  that 
they  do  not  advance  in  any  direction  indiscriminately, 
as  does  a  jelly-fish,  but  that  there  is  one  end  which 
is  always  directed  forwards,  and  which  first  comes  into 
contact  with  friends,  foes,  or  food.  It  is  at  that  end, 
naturally,  that  sense  organs  are  first  and  best  deve- 
loped, and  it  is  at  that  end,  therefore,  that  the  central 
portion  of  the  nervous  system  comes  to  be  largest  and 
most  highly  developed. 

In  connection  with  this,  the  discovery  by  Kleinen- 
berg  of  a  nervous  ring  in  the  larvae  of  certain  Annelids 
is  of  great  significance  ;  for  though  the  adult  poly- 
chsetous  worm  is  bilaterally  symmetrical,  and  has  a 
central  nervous  system  of  the  same  character,  the 
larva  has  a  rounded  head-disc. 

After  the  disappearance  of  the  diffused  or  plexi- 
forui  arrangement  of  the  nefVe  fibres  the  system  may 


404  COMPARATIVE  ANATOMY  AND.  PHYSIOLOGY. 

fetill  retain  a  very  close  connection  with  the  surface 
bi  the'  body  ;  the  Annulata,  for  example,  present  us 
with  various  arrangements  of  this  kind,  for  while 
Chsetopterus  and  Spio  have  the  nerve  cords  out- 
side the  muscular  layer  of  the  body  wall,  and  others, 
such  as  Hermella,  have  them  between  or  even  in 
the  substance  of  these  muscles,  others  again,  like 
the  earthworm,  have  them  placed  inside  the  muscular 
layers. 

In  the  simplest  condition  of  those  form's  which  do1 
not  present  the  most  primitive  arrangements^  we  find 
a  central  gaiiglionic,  or  cerebral  mass*  with 
which  there  are  connected  a  number  of  nerve  fibres, 
which  pass  to  different  parts  of  the  body  ;  such  «i 
disposition  is  found  in  some  of  the  Turbellaria,  and  in 
the  .Rotatoria. 

The  most  important  advance  is  seen  in  the  appear- 
ance of  the  main  or  longitudinal  cords,  such  as  we  have 
already  noted  in  Peripatus ;  but  even  when  these  do 
appear,  we  find  that  the  cerebral  mass  still  gives  off  a 
number  of  fibres,  which  pass  to  the  different  sensory 
organs  that  are  situated  at  the  anterior  end  of  the 
body.  The  two  main  trunks  that  pass  backwards  are 
more  or  less  intimately  connected  with  one  another  on 
the  ventral  surface  of  the  gullet,  so  that  we  have  now 
to  distinguish  the  carebral,  or  supracBsophageal 
ganglia,  the  cesopliageal  nerve  cords  or  com- 
missures, and  the  sufooesopliageal  ganglia; 
these  last  are,  in  their  most  primitive  conditions, 
similar  to  those  that  follow  them  (Fig.  174)  ;  at 
first  they  are  not  closely  united  with  one  another, 
but  connected  together  by  a  pair  of  transverse  com- 
missural  cords,  as  are  the  ganglia  that  follow  them. 

In  the  more  primitive  conditions,  such  as  are 
presented  by  Apus  among  the  Crustacea,  the  cerebral 
ganglia  are  merely  formed  by  the  nervous  swel- 
lings in  the  anterior  region  (primitive  cerelrum) 


Chap,  xi  r.  ]     NER  vo  us  SYS  TEM  OF  An  THRO  POD  A  . 


405 


("  archicerebrum"  of  Lan- 
kester).  Such  an  arrange- 
ment is  found  also  among 
annulate  worms. 

In  the  greater  number 
of  the  Arthropoda  we 
not  only  see  that  the  nerve 
trunks  lie  internally  to  the 
muscular  layers  of  the 
body  wall,  but  also  that 
the  cerebrum  is  no  longer 
primitive,  but  has  other 
ganglionic  cells  used  with 
it ;  or,  to  use  the  words  of 
Rathke,  as  applied  to  the 
developing  scprpion,  the 
brain  is  "  composed  of 
several  pairs  of  ganglia 
lying  one  behind  the 
other."  Nor  is  this  kind 
of  fusion  confined  to  the 
brain  ;  a  longitudinal  sec- 
tion of  part  of  the  nerve 
cords  of  a  crayfish  shows 
that  the  ganglionic  cells  in 
a  segment  have  become 
closely  united  together, 
while,  at  the  same  time, 
the  cords  are  still  distinct. 
Nor  is  this  all  ;  while 
Apus  has  a  distinct  gang- 
lionic enlargement  in 
every  segment  of  its  body, 
we  find  that  in  higher 
forms  various  ganglia  be- 
come connected  together, 
until  at  last,  in  the  common 


0(7 


Fig.  174— Diagram  of  the  Ante- 
rior Portion  of  the  Nervous 
System  of  Apus,  showing  the 
"  archicerebrum  "  (c),  and  the 
Ganglia  of  the  Lateral  Cord?. 
(From  Lankester.  after  Zad- 
dach.) 

x,  Frontal  nerves  ;  oc,  optic  nerves  ; 
CE,  oesophagus  ;  a  1.  nerve  for  first 
antenna;«2,  nerve  for  serondditto  : 
Mri,  nerves  for  mandible  ;  MX,  for 
maxilla;  wp,  for  maxllliped;  T  1, 
for  first  thoracic  appendage. 


406  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


crab,  all  the  ganglia  behind  the  cerebral  become  fused 
into  one  large  mass,  which  still  retains  evidence  of  its 
composite  character  by  giving  off  a  large  number  of 
separate  nerve  fibres  (Fig.  175).  A  similar  series  illus- 
trating the  phenomenon  of  the  fusion  of  nerve  centres 
may  be  observed  jn  Araolniida  and  Insecta. 

We  note,  then,  that  the 
loss  of  that  plexiform  arrange- 
ment, of  which  we  have 
spoken  so  often,  i§  succeeded 
by  an  aggregation  of  ganglionic 
cells,  which  form  distinct 
masses  in  every  segment  of 
the  body  ;  at  first  each  mass 
is  composed  of  two  distinct 
halves.  The  anterior  regioD. 
becomes  more  and  more  pre- 
dominant, and  the  "  archicere- 
brum,"  or  simple  anterior 
enlargement,  becomes  a  "  syn- 
perebrum,"  or  compound  one.. 
As  the  segments  of  the  body, 
which  in  the  earthworm,  for 
example,  are  all  alike  and  ha.ve 
nearly  all  just  the  same  func- 
tions, become  arranged  in 
groups  which,  as  in  the  cray- 
fish, take  on  different  duties,  or  exhibit  division  of 
labour,  the  nervous  centres  likewise  become  affected, 
so  that  while  Apus  has  a  separate  ganglionic  mass 
for  each  of  its  sixty  segments,  the  crayfish  has  the 
first  six  of  its  ventral  ganglia  fused  together,  and 
the  short-tailed  crab  has  all  the  ventral  ganglia  in  a 
single  mass  (Fig.  175);  so,  again,  the  Myriopod  has 
ganglia  in  every  one  of  its  segments,  the  scorpion 
has  the  first  nine  ventral  ganglia  united,  and  in  the 
short-bodied  spider  there  is  only  one  ventral  ganglion. 


Fig.   175.— Nervcras    System 
of  a  Crab. 

c,  Cerebral  ganslion;  o,  optic; 
a.  antennary  nerye  ;  c,  O?PO- 
phageal  commissure,  T,  fused 
ventral  ganglion. 


chap,  xii.]     NER  vous  SYSTEM  OF  ECHINODERMS;  407 

We  meet  with  the  same  phenomenon  in  Insects, 
*but  these  Arthropods  are  of  greater  interest  from 
the  point  of  view  that  the  changes  undergone  by 
them  during  their  development  afford  support  to  the 
view  that  the  more  primitive  forms  have  a  larger,  and 
the  more  complex  a  smaller,  number  of  separate 
ganglia.  While  the  worm-like  larva  has  a  ganglionic 
mass  in  nearly  every  one  of  its  segments,  the  adult 
insect  has  a  varying  number  fused  together. 

As  has  been  already  pointed  out  in  speaking  of  the 
Ectimodermata,  the  nervous  system  of  a  starfish 
is  so  far  extremely  primitive  in  character,  that  the 
nerve  cord  which  runs  down  the  ainbulacral  groove  of 
each  arm  lies  just  below  the  integument  ;  in  the 
Ophiuroids  this  superficial  position  is  lost,  owing  to  the 
development  of  a  calcareous  plate,  which  forms  a  floor 
for  the  groove.  The  great  development  of  the  test  in 
Echinoids  leads  to  tl*e  same  result ;  but  here,  as  we  have 
already  learnt,  a  compensating  arrangement  is  effected 
by  the  development  of  a  plexus  of  nerve  cells  and 
fibres  which  is  superficial  to  the  test.  In  Holo- 
thurians  the  nerve  cords  are  placed  more  or  less  deeply 
in  various  forms. 

In  all  cases  these  radially  disposed  nerve  cords  are 
united  with  one  another  by  a  set  of  circular  fibres, 
which  form  the  circumoral  nerve  ring,  and  it  is 
thanks  to  this  that  the  apparently  independent  rays 
of  a  starfish  or  of  a  brittle  star  are  enabled  to  act  in 
concert :  but,  although  the  nervous  system  of  an  Echi- 
noderm  is  hereby  made  a  connected  whole,  it  is  im- 
portant to  observe  that  a  single  arm  of  a  starfish,  or 
even  a  segment  of  an  Echinus  (Fig.  176)  is  capable  of 
exerting  independent  movements  ;  for  example,  single 
rays  of  a  starfish  have  been  found  to  crawl  as  fast, 
and  in  as  definite  a  direction  as  entire  forms  ;  if  turned 
on  their  back  they  succeed  in  righting  themselves,  and 
sometimes,  though  not  always,  they  attempt  to  move 


408    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

away  from  injuries  inflicted  on  them.  If  the  nerve 
ring  be  divided,  without  the  separation  of  a  ray  from 
the  rest  of  the  organism,  the  ray  whose  nervous  con- 
nection is  so  cut  ceases  to  act  with  the  rest  of  the  star- 
fish, but  is  capable,  to  a  certain  extent,  of  responding 
to  stimuli  on  its  own  account. 


Fig.  176.— Separate  Segment  of  an  Echinus  attempting  to  right  itself 
.  after  having  been  inverted.     (After  Komanes  and  Ewart.) 


The  Crinoidea  must  be  dealt  with  separately  from 
the  rest  of  the  Echinoclermata,  in  consequence  of  the 
difficulties  presented  by  the  conditions  and  relations 
of  their  nervous  system.  When  a  transverse  section 
is  made  of  one  of  the  pinnules  which  hang  down  from 
the  sides  of  an  arm  of  a  Crinoid,  a  nerve  cord  (n ;  Fig. 
177)  is  seen  to  lie  just  underneath  the  epithelium  of 
the  groove  of  the  pinnule  ;  this  clearly  corresponds  to 


Chap,  xii.]     NERVOUS  SYSTEM  OF  CRINOIDS. 


409 


a.' 


the  nerve  in  the  arm  of  a  starfish,  and  it  has  similar 
relatio'ns  to  a  nerve  ring  which  runs  round  the  mouth. 
If  we  now  look  at  the  opposite  side  of  the  section,  we 
find  another  and  larger 
cord  which  gives  oft 
branches  to  the  muscles 
of  the  arms  (a  a)  ;  this 
cord,  if  it  be  followed 
up,  will  be  found  to 
end  in  an  organ,  the 
so  -  called  "  chambered 
organ,"  which  lies  in 
the  centred  orsal  piece 
(see  page  292)  of  the 
Crinoi('.  Now,  if  the 
visceral  mass,  part  of 
which  is  the  circumoral 
nerve  ring,  be  alone 
removed,  the  arms  will 
continue  to  move  as 
regularly  as  they  did 
before,  and  the  Crinoid 
will  still  be  able  to 
swim  about  in  the 
water.  If,  on  the  other 
hand,  the  five  -  cham- 
bered organ  be  stimu- 
lated, then,  as  Dr. 
Carpenter  has  shown, 
there  is  a  sudden  and 
simultaneous  flexion  of 
all  the  arms.  The  ex- 
istence of  these  two 

apparently  independent  nerve  systems  in  a  Crinoid 
is  a  difficulty  which  the  morphologist  has  not  yet 
been  able  to  solve,  but  the  anatomical  and  physio- 
logical evidence  in  favour  of  the  nervous  nature 


CL 


Fig.  177.— Cross  Section  of  a  Pin- 
«ule  of  the  Arctic  Feather- 
star  (Antedon  (schncJiti) ;  x  75. 

a,  Axial  cord  ;  a',  its  branches ;  ag,  ambu- 
lacral  groove;  b,  radial  blood- 
vessel ;  gv,  genital  vessel ;  ov,  ovary  ; 
•n,  radial  nerve  ;  pj,  pinnule  joint ; 
w,  water-vessel ;  T,  tentacle.  (From 
Carpenter,  altered  from  Ludwig.) 


4i o  CoMPARAi^iy-E  ANATOMY  AND  PHYSIOLOGY. 

of  the  chambered  organ  and  the  axial  cords  appears 
to  be  complete.* 

The  greater  number  of  the  Mollusca  present  us 
with  an  arrangement  of  the  nervous  system  which  is 
very  different  from  that  which  obtains  in  Arthropods ; 
this  is  due  to  the  want  of  metameric  segmentation, 
and  to  the  marked  tendency  of  the  ganglionic  masses 
to  fuse  with  one  another.  Indications  of  a  more 
primitive  condition  of  things  are  not,  however  con- 
fined to  Proneomenia  (page  402);  commissures  con- 
necting the  two  chief  longitudinal  trunks,  and  so 
giving  rise  to  a  step-ladder-like  kind  of  arrangement, 
are  to  be  observed  in  Chiton  and  in  Haliotis. 

In  the  L,amelliforanchiata  (e.g.  Anodon),  when 
the  primitive  bilateral  symmetry  of  the  body  is  re- 
tained, we  find  two  supraoesophageal  ganglia,  whence 
nerve  cords  pass  off  on  either  side  to  the  hinder  end  of 
the  body ;  no  ganglia  are  developed  on  the  course  of 
these  trunks,  but,  as  in  Proneomenia,  at  their  termin- 
ations only  (visceral  ganglia)  ;  these  two  ganglia 
are  sometimes  almost  separate,  in  other  cases  more  or 
less  completely  fused  with  one  another,  just  as,  at  the 
other  end  of  the  body,  is  the  case  with  the  supra- 
oesophageal ganglia. 

These  last  also  give  off  a  pair  of  cords,  which  in  the 
mussel  extend  some  way  down  into  the  substance  of 
the  foot,  where  they  end  in  the  pedal  ganglia ;  but 
these  pedal  ganglia  are  not  always  so  far  distant  from 
the  supracesophageal  as  in  the  mussel,  their  size  and 
position  depending  on  that  of  the  foot  itself. 

While  the  supracesophageal  or  cerebral  ganglia  of 

*  Prof.  Milnes  Marshall,  who  has  lately  repeated  and  extended 
the  observations  of  Dr.  Carpenter,  has  suggested  that  the  ant- 
ambulacral  or  dorsal  portion  of  the  nervous  system  of  a  Crinoid  is 
modified  from  the  antambulacral  portion  of  the  primitive  nerve 
sheath,  which  in  the  starfish  still  invests  the  whole  of  the  body. 
The  "chambered  organ,"  or  "central  capsule,"  still  requires  in- 
vestigation from  the  morphological  and  embryological  side.. 


chap,  xii.]     MOTOR  AND  SENSORY  NERVES.          411 

the  Lamellibranchiata  (Acephala)  are  always  com- 
paratively small,  in  consequence  of  the  reduction  of 
the  head  of  these  Molluscs,  they  are  always  much 
larger  in  the  Cephaloptiora,  which  are  provide^ 
with  eyes  and  powerful  £acti}e  tentacles.  The  two 
most  important  phenomena  observable  jn  the  charac- 
ters of  the  nervous  system  ,of  this  group  are  th,e  fusion 
of  the  primitively  separate  ganglionj.c  masses,  and  the 
twisting  undergone  by  the  nerve  cords  of  soine  of  the 
Gastropoda.  The  former  attains  its  most  marfceql  de- 
velopment in  the  Cephalopoda,  where  the  pedal  fuse 
with  the  visceral  ganglia,  and  are  closely  approxi- 
mated to  the  cerebral  mass  ;  the  latter,  which  may  be 
seen  in  the  limpet  (Patella),  or  the  river-snail  (Palu- 
dina),  results  in  the  nerves  which  connect  the  cerebral 
with  the  visceral  ganglia  passing  from  the  right  to 
the  left,  a#d  from  the  left  to  the  right-hand  side. 

From  the  ganglionic  masses  and  from  the  cords  that 
connect;  £hem  together  m  the  way  that  }ias  now  been 
described,  nerves  are  giyen  off  to  various  parts  of  the 
body.  We  have  already  seen  that  in  the  lower  forms 
the  whole  of  the  body  is  invested  in  a  superficial 
plexus  qf  nerve  fibres  and  cells  ;  as  the  cells  became 
gradually  aggregated  intp  plefinjte  mass^s,  the  n,erves 
that  were  given  off  from  then}  became  likewise  arranged 
in  a  defjnjte  and  regular  fashion}  and  took  on  definite 
duties,  and  functions.  Those  nerves  jthat  pass  to 
muscles  may  be  spoken  of  as  the  motor  pr  efferent 
nerves,  those  that  end  in  sensory  organs,  whether 
general  tactile  organs  or  organs  of  more  espepia}  sense, 
as  sensory  or  afferent  nerves ;  that  is  tp  say,  they 
bring  messages  to  the  central  system,  while  the  efferent 
nerves  carry  messages  away.  The  size  and  number  of 
these  nerves  depend,  therefore,  primarily  on  the  size 
of  the  parts  to  which  they  are  distributed.  Their 
general  arrangement  may  be  well  seen  in  a  segmented 
animal  ;  putting  aside  for  a  moment  the  nerves  given 


412    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

off  from  the  supra cesophageal  ganglia,  we  find  that  in 
the  earthworm,  for  example,  several  nerves  are  given 
off  from  the  cesophageal  commissures,  and  that  each 
successive  ganglion  gives  off  two  nerves  on  either  side, 
while  one  nerve  on  either  side  is  given  off  by  the  cords 
which  connect  the  ganglia  witl)  one  another.  When 
we  come  to  a  more  differentiated  form,  si^ch  as  the  cray- 
fish, we  find  that  no  nerves  are  given  off  from  the 
commissures,  but  that  three  pairs  of  nerves  are  sent  ofjf 
from  each  of  tfye  ganglia  that  belong  to  one  segment 
only,  while,  when  two  op  more  ganglia  have  fused 
together,  a  large  number  qf  nerves  are  given  off  ii) 
order  to  supply  more  than  one  segment  of  the  body. 

In  addition  tq  the  sensory  and  motor  nerves  therp 
are  others  which  are  particularly  related  to  the  digesr 
tive  and  cirpulatqry  organs ;  the§e  are  the  so-calLed 
visceral  nerves,  and,  from  a  physiological  standpoint, 
if  no£  indeeq1  also  froin  a  morphological,  they  aro 
comparable  tq  the  system  which,  in  Man  and  other 
vertebrates,  is  spoken  of  as  the  sympathetic  system. 
While  in  thje  lower  worms  these  yisperaj  nerves  are 
merely  cords  given  off  frqm  tl^e  cerebral  ganglia,  they 
become  more  independent  in  the  higher  forms,  owing 
to  the  development  of  ganglia  along  their  course  ;  a 
well-marked  ganglion  of  this  kind  may  be  seen  on  the 
dorsaj  surfape  of  the  crop  of  the  cockroach.  The 
general  arrangement  of  the  •'  stomatorgastric  "  system  of 
this  animal  wi}i  serve  conveniently  as  a  type,  and  may 
"be  thus  describe.4  ;  from  the  anterior  part  of  the  cere- 
bral mass  a  cord  arises  on  either  side,  which,  after 
passing  forwards  for  a  short  distance,  bends  on  itself 
and  unites  with  its  fellow  in  a  median  ganglion.  The 
single  cord  given  off  from  this  ganglion  passes  back- 
wards beneath  the  brain  to  .another  median  ganglion  ; 
with  this  last  two  lateral  ganglia  are  connected  ;  the 
second  median  ganglion  gives  off  a  cord  which  passes 
backwards  above  the  digestive  tract  to  a  third  ganglion 


chap,  xii.]    NERVOUS  SYSTEM  OF  CRAYFISH.       413 

or  that  already  mentioned ;  from  this  there  arise  two 
trunks  which  give  oft'  nerve  fibres  to  the  anterior  por- 
tions of  the  digestive  tract.  While  the  median 
ganglia  and  nerves  form  the  unpaired  system,  the  two 
lateral  ganglia  are  the  most  anteriorly  placed  repre 
sentatives  of  a  paired  system  of  stomato  -  gastric 
nerves  and  ganglia. 

Other  nerve  cords  connected  with  the  sympathetic 
system  supply  especially  the  air  tubes  (trachese),  and 
the  muscles  of  their  Orifices  (stigmata)  ;  from  the  fact 
that  the  nerve  which  runs  above  the  ventral  gangli- 
onic  chain  gives  off  lateral  branches  which  pass  out- 
wards, the  system  is  known  as  that  of  the  nervi 
transversi  accessorii.  In  the  crayfish  the  ter- 
minal ganglion  of  the  ventral  chain  gives  off'  nerve 
fibres  which  innervate  the  hinder  portion  of  the 
digestive  tract. 

The  function  of  the  several  parts  of  the  nervous 
feystenl  have  been  investigated  in  so  few  of  the  Inver- 
tebrata,  that  it  will  be  well  to  state  at  some  length 
what  is  definitely  known  as  to  the  physiology  of  the 
nervous  system  of  the  crayfish  or  the  lobster. 

We  note  in  the  first  place  that  the  presence  of  a 
comparatively  large  cerebral  mass  is  associated  with  a 
large  amount  of  influence  over  the  rest  of  the  ganglia; 
thus,  the  limbs,  which  in  ordinary  circumstances  move 
in  due  order  in  such  a  way  as  not  to  oppose,  but  rather 
to  assist  one  another,  cease  to  exhibit  this  harmonious 
activity  when  the  cerebral  ganglia  are  removed  \  in 
other  words,  they  are  no  longer  co-ordinated  ;  but  this 
is  not  all  ;  the  cerebrum  appears  to  be  the  centre  of 
what,  in  our  ignorance  of  all  the  circumstances  of  the 
case,  we  call  spontaneous  activity,  and  this  is  very 
pointedly  spoken  to  by  the  loss  of  power  in  the  selec- 
tion of  food,  which  follows  on  a  removal  of  the  cerebral 
centres.  The  separate  condition  of  the  resophageal 
commissures  which  unite  the  brain  with  the  chain  of 


414  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

ventral  ganglia  is  not  only  an  anatomical  fact,  it  has 
also  a  physiological  significance,  for  when  that  of  one 
side  is  removed,  it  is  only  the  organs  on  that  side  of  the 
body  which  cease  to  react  to  stimuli,  the  appendages 
on  the  other  side  alone  appearing  to  be  affected. 

The  ganglia  just  below  the  O3sophagus  (the  sub- 
O3sophageal)  appear  to  have  a  considerable  function 
as  the  centres  of  motor  energy,  for  so  long  as  they  are 
present  the  appendages  move  with .  considerable  ac- 
tivity, but  when  they  are  removed  the  chelae  "  sprawl 
helplessly,"  and  the  legs  are  often  found  doubled  up 
under  the  body.  As  might  be  supposed  from  the  re- 
lations of  their  nerve  fibres  to  the  muscles  of  the 
gnathites,  the  same  ganglia  appear  to  be  the  centre 
for  the  feeding  movements  ;  after  their  extirpation, 
the  chelae  or  great  forceps  do  not  always  carry  the 
food  to  the  mouth,  as  they  do  regularly  in  the  uninjured 
animal ;  it  is  a  curious  fact  that  even  when  they  do 
carry  it  there  they  do  not  give  it  up  to-  be  swal- 
lowed. 

With  regard  to  the  general  physics  of  the  nerve 
fibres,  we  know  from  Fredericq  that  motor  excitations 
produced  by  electrical  currents  pa,ss  much  more  slowly 
along  the  motor  nerve  of  a  lobster  than  that  of  a 
frog,  the  proportion  per  second  being  as  twenty-seven 
metres  in  the  frog  to  six  in  the  lobster. 

The  student  of  vertebrate  physiology  will  best 
understand  the  leading  differences  between  the  ac- 
tivities of  the  nervous  system  of  the  frog  and  of  the 
crayfish,  by  a  comparative  statement  :  "  There  is  much 
less  solidarity,  a  much  less  perfect  consensus  among 
the  nervous  centres  in  the  crayfish  than  in  animals 
higher  in  the  scale.  The  brainless  frog,  for  example, 
is  motionless  except  when  stimulated,  and  even  then 
does  nothing  to  suggest  that  its  members  have  a  life 
on  their  own  account ;  whereas  the  limbs  of  a  cray- 
fish, deprived  of  its  first  two  ganglia,  are  almost 


chap  xii.]    NERVOUS  SYSTEM  OF  CHORDATA.       415 

incessantly  preening,  and,  when  feeding  movements 
are  started,  the  chelate  legs  rob  and  play  at  cross  pur- 
poses with  each  other  as  well  as  four  distinct  indivi- 
duals could  do  "  ( J.  Ward). 

This  quotation  will  bring  very  forcibly  to  the 
mind  the  value  and  meaning  of  gangl  ionic  masses  in 
the  separate  segments. 

So  far  as  our  present  knowledge  extends,  we  are 
led  to  the  belief  that  the  spinal  cord  of  the  lower 
Vertebrates  (as  represented  by  the  frog)  has  much 
greater  independence  than  that  of  the  higher,  as  re- 
presented by  the  dog,  or  by  man.  For  example,  if 
the  brain  of  a  frog  be  removed,  the  animal  will  still 
execute  movements,  to  which  it  is  impossible  to  re- 
fuse the  name  of  purposeful ;  in  the  Mammal,  on 
the  other  hand,  the  movements  which,  under  similar 
conditions,  are  similarly  excited,  are  irregular  and 
without  order.  Extirpation  of  the  cerebral  hemi- 
spheres of  a  Mammal  results  in  death  after  a  few 
hours,  while  the  frog  may  be  kept  alive  for  an  indefi- 
nite period,  if  suitable  care  be  taken  of  it. 

The  general  functions  of  the  various  parts  of  the 
brain  have  been  discussed  in  the  volume  on  "Human 
Physiology"  (chap.  xiv.). 

The  Chordata  are  to  be  distinguished  practically, 
even  if  not  morphologically,  from  the  majority  of  the 
so-called  Invertebrata  by  the  fact  that  the  nervous  cord 
lies  on  the  dorsal  aspect  of  the  body,  and  not  on  that 
on  which  the  mouth  is  situated;  at  the  same  time 
it  is  to  be  borne  in  mind  that  in  the  Nemertinea  the 
nerve  cords  often  tend  to  lie  dorsally,  and  that  in 
Peripatus  the  two  cords  are,  at  the  hinder  end  of  the 
body  connected  together  by  a  commissure  which  lies 
above  or  dorsally  to  the  terminal  portion  of  the  in- 
testine. Similarly,  there  are  certain  points  in  the 
anatomy  of  the  vertebrate  brain,  too  complicated  to 
be  here  described,  which  afford  some  evidence  in 


416    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

favour  of  the  view  that  the  anterior  portion  of  the 
brain  was  once  separated  from  that  which  lies  behind 
it  by  the  digestive  tract. 

In  no  known  Chordate,  however,  does  the  oeso- 
phagus separate  any  one  part  of  the  nervous  system 
from  the  rest,  and  the  whole  mass  is  superior  to  or 
dorsal  in  relation  to  it.  In  all  Chordata  also  the 
nerve  cord  has  a  central  canal,  and  occupies  exactly 
the  median  axis  of  the  body.  The  presence  of  this 
canal  is  not  to  be  explained  without  a  reference  to  the 
history  of  the  development  of  the  central  nervous 
system  ;  in  this  mode  of  development  we  find  yet 
another  important  characteristic  of  the  Chordata. 

The  median  strip  of  epiblast  which  is  to  give  rise 
to  the  nerve  cord,  instead  of  merely  sinking  away  from 
the  surface  of  the  body,  becomes  grooved  along  its 
middle  line ;  the  sides  of  the  groove  grow  up  and 
unite  with  one  another,  so  as  to  leave  a  central  cavity  ; 
in  most  cases  the  tube  is  first  formed,  and  only  later 
on  separates  off  from  the  layer  of  epiblastic  cells 
which  forms  the  covering  of  the  body  ;  in  Amphioxus, 
however,  the  external  layer  covers  over  the  so  called 
"  medullary  plate  "  which  forms  the  nerve  cord  before 
the  groove  has  become  closed  up.  It  will  be  seen 
that,  owing  to  the  formation  and  closure  of  this 
groove,  the  cells  that  were  primitively  external  come 
to  lie  within  those  that  were  primitively  internal. 

In  the  Cephalocliordata  the  central  nervous 
system  retains  throughout  life  the  form  of  a  hollow 
tube,  and  there  is  no  distinct  enlargement  at  the  anterior 
end  which  can  be  called  a  brain.  In  the  U  roeliordata 
the  typical  arrangement  is  best  seen  in  those  which 
retain  the  tail  during  the  whole  of  their  lives  (Appen- 
dicularia)  ;  in  them  we  find  an  anterior  swelling, 
which  becomes  divided  into  two  vesicles,  with  the 
foremost  of  which  an  optic  and  an  auditory  organ 
become  connected ;  the  hinder  vesicle  is  separated  by 


Chap.  XII.] 


BRAIN  OF  VERTEBRATA. 


CH- 


CV- 


N.. 


-H3 


a  constriction  from  the  cord  that  follows  it.  and  from 
which  three  pairs  of  nerves  have  been  observed  to  be 
given  off.  In  this  cord,  as  in  that  of  the  Yertebrata, 
we  find  that  the  nerve  fibres  lie  externally  to  the  gang- 
lionic  cells,  an  arrangement  of  the  histological  elements 
which  is  exactly  the  reverse  of 
what  obtains  in  "invertebrates." 

With  the  loss  of  the  tail,  the 
nerve  cord,  which  is  found  in  the 
tailed  larva  in  the  same  position 
as  in  the  adult  Appendicularia, 
undergoes  atrophy,  and  the  fixed 
or  colonial  Tunicate  has  a  single 
ganglionic  mass  which  lies  be- 
tween the  mouth  and  the  atrio- 
pore.  (See  page  231.)  From 
this  ganglion  nerves  are  given 
off  to  the  different  parts  of  the 
body. 

In  the  Vertebrata  a  brain 
is  always  present ;  the  primi- 
tively single  swelling  at  the  an- 
terior end  rapidly  becomes  divided 
into  three  brain  vesicles,  which 
may  be  distinguished  as  those  of 
the  fore-,  mid-,  and  hind-  brain. 
These  vesicles  are,  of  course,  hol- 
low within,  and  their  cavities 
have  received  distinct  names,  the 
reasons  for  which  will  certainly 
be  far  from  clear,  unless  we  recollect  that  the  termin- 
ology of  the  parts  of  the  vertebrate  brain  is  based  on 
the  nomenclature  of  anthropotornists.  The  cavity  in 
the  fore-brain  (Fig.  178;  m)  is  known  as  the  third 
ventricle,  and  that  in  the  hind-brain  (iv)  as  the 
fourth  ventricle  ;  the  often  narrower  cavity  in  the 
mid -brain  (it)  is  known  as  the  iter  a  tertio 
BB— 16 


Fig.  178.— Diagram  of  the 
Ventricles  of  the 
Vertebrate  Brain. 

in,  third  ventricle ;  it,  iter ; 
iv,  fourth  ventricle  ;  CH, 
cerebral  hemispheres; 
cv, their  cavity ;  FM, fora- 
men of  Munro  ;  FB,  f ore- 
brain  ;  JIB,  mid-brain  : 
HB,  hind-brain. 


4i 8  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

ad    quartum   ventriculiim,  or  more  shortly  as 
the  iter. 

The  walls  of  these  cavities  undergo  further 
changes ;  the  hind-brain  becomes  divided  into  two 
parts,  one  of  which  lies  behind,  and  at  a  little 
lower  level  than  the  other;  this  is  the  medulla 
oblongata,  and  it  is  directly  continuous  with 
the  spinal  cord*  The  anterior  half,  which  in  the 
frog  is  a  narrow  band,  but  in  man  forms  a  very 
conspicuous  part  of  the  whole  mass,  is  the  so-called 
little  brain  or  cerebellum.  The  mid-brain  does 
not  undergo  transverse  division  ;  its  upper  and  late- 
ral portions  form  the  optic  lobes,  and  the  inferior 
portion  the  so-called  crura  cerebri.  The  most  re- 
markable changes  are  undergone  by  the  fore-brain 
vesicle,  which  buds  out  a  vesicle  on  either  side,  the 
cavities  in  which  are  known  as  the  lateral  ventricles 
(cv) ;  these  lateral  outgrowths  always  become  of  con- 
siderable size,  and  in  the  higher  vertebrates  form  the 
chief  mass  of  the  brain.  They  are  the  cerebral  hemi- 
spheres, and  are  the  seat  of  the  most  important  of 
the  functions  performed  by  the  brain ;  they  not  only 
increase  in  size,  but  by  the  development  of  grooves, 
the  presence  of  which  permits  an  addition  to  the 
quantity  of  grey  or  ganglionic  material  altogether 
out  of  proportion  to  the  increase  in  the  area  occupied, 
they  come  to  have  not  only  a  more  complicated  sur- 
face, but  also  a  much  higher  functional  value. 

The  cerebral  hemispheres  are  continued  anteriorly 
into  the  olfactory  lobes  (Fig.  179  ;  o£),  and  these 
into  the  so-called  olfactory  nerves.  More  pos- 
teriorly, the  fore-brain  gives  off  another  vesicle  on 
either  side,  and  this  vesicle  travels  away  from  the 
brain,  with  which  it  only  remains  connected  by  its 
stalk ;  the  vesicle  forms  the  hinder  part  of  the  eye, 
and  the  stalk  becomes  the  so-called  optic  nerve. 
The  remainder  of  the  fore-brain  forms  the  thalamen- 


Chap.  XII.] 


BRAIN  OF  FROG. 


419 


cephalon,  or  optic  tlialami,  so  called  from  the  fact 
that  when  the  brain  is  laid  on  its  upper  surface  the 
optic  nerves  rest  on  them  as  on  a  couch  (thalamus). 

Connected  with  the  upper   surface  of  the  thala- 
mencephalon   is   the   pineal  gland,    which   is  not 


01 


CH 


Fig.  179.— A,  Brain  of  Frog  from  above  ;  B,  from  below. 

1,  Olfactory  nerves;  ol,  olfactory  lobes;  CH,  cerebral  hemispheres;  i-T, 
lamina  terminalis  ;  T^,  thalamencephalon  with  pineal  gland  (PG)  ;  OpL,  optic 
lobes;  cl,  cerebellum;  MO,  medulla  oblongata ;  2,  9ptic  nerves ;  OT,  optic 
chiasma  ;  TC,  tuber  cinereum ;  H,  hypophysis  cerebri ;  3—10,  cerebral  nerves. 
(After  Ecker.) 

nervous  in  nature,  while  the  lower  surface  of  the 
same  region  of  the  brain  is  continued  into  the  funnel- 
shaped  tnfoer  cinereum  (Fig.  179  ;  TO),  with  the 
base  of  which  is  connected  the  so-called  pituitary 
body;  this,  like  the  pineal  gland,  is  not  nervous  in 


420  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

nature,  and  is  a  structure  which  is  not  of  cerebral 
origin  at  all,  but  is  derived  from  the  epiblast  which 
lines  the  cavity  of  the  mouth  ;  in  its  primitive  con- 
dition it  forms  an  inpushing  towards  the  lower 
surface  of  the  brain ;  its  base  becomes  solid,  and 
then  disappears,  so  that  the  ingrowth  becomes 
completely  separated  off  from  the  layer  of  cells 
from  which  it  took  its  origin.  In  the  lower  ver- 
tebrates it  does  not,  but  in  mammals  it  does,  become 
structurally  united  with  the  brain. 

In  Fishes  the  brain  is  always  small ;  in  the  pike, 
for  example,  it  is  not  more  than  T^^th  part  of  the 
weight  of  the  whole  body,  whereas  in  Man  it  is  about 
•g^th  of  the  total  weight ;  nor  does  it  grow  propor- 
tionately with  the  growth  of  the  body,  or  occupy  the 
whole  of  the  cranial  cavity.  In  the  Cyclostomata 
the  walls  of  the  cerebral  hemispheres  become  greatly 
thickened,  so  much  so,  indeed,  that  in  Myxine  they 
become  quite  solid ;  the  olfactory  lobes  are  propor- 
tionately large,  as  is  also  the  pineal  gland ;  the 
region  of  the  hind-brain  is  also  of  great  size,  propor- 
tionately to  the  rest  of  the  organ.  In  the  Elasmo- 
branchs  the  olfactory  lobes  are  often  carried 
forwards  on  stalks,  which  are  of  great  length  in  some 
sharks  ;  these  lobes  may  be  broken  up  into  smaller 
lobules.  The  cerebral  hemispheres  are  proportionately 
large,  and  differ  greatly  in  the  size  of  the  contained 
ventricles,  or,  in  other  words,  in  the  thickness  of  their 
walls  ;  the  surface  of  these  hemispheres  is  sometimes 
marked  by  a  few  shallow  grooves.  The  cerebellum  is 
of  large  size,  and  is  often  grooved  transversely. 

During  the  process  of  development  the  brain 
vesicles  cease  to  lie  in  a  straight  line  one  behind 
the  other ;  as  a  consequence  of  this  "  cranial 
flexure,"  the  fore-brain  lies  at  a  lower  plane  than 
the  mid- brain,  and  the  long  axes  of  the  two  are 
set  at  an  angle  to  one  another.  A  little  later  the 


chap.  xii. i   BRAINS  OF  FISHES  AND  AMPHIBIA.    421 

wal)  between  the  two  parts  of  the  fore-brain  becomes 
thinner,  and  a  "  primitive  cerebral  fissure  "  is  apparent. 
This  condition  of  things  is  retained  by  some  Ganoids 
throughout  life  (Polypterus  ;  Fig.  180);  these  fishes 
also  possess  the  more  primitive  character  of  a  large 
thalamencephalon. 

In  the  Teleosfei  the  brain  is  compressed,  the 
cerebral  hemispheres  are  almost  completely  solid,  and 
the  cerebellum  is  usually,  though  not  always,  of  com- 
paratively large  size  ;  it  is  often  prolonged  into  the 
cavity  of  the  mid-brain  (valvula  cerebelli) ;  on  the 


Fig.  ISO.— Brain  of  Polypteriis  seeu  from  the  Side, 

t  Olfactory  nerves  ;  h,  k,  cerebral  hemispheres  •  o,  optic  nerve  ;  d,  optic  lobes  ; 
e,  hypophysis  ;/r  central  fissure  ;  6,  c,  cerebellum;  a,  mednlla  oblongata.  (After 
J.  Miiller.) 


whole,  the  brain  of  the  Teleostei  exhibits  many  resem- 
blances to  that  of  Ganoids,  and  especially  of  Lepi- 
closteus. 

In  the  adult  Amphibia,  as  in  the  adults  of  most 
fishes,  the  several  parts  of  the  brain  lie  in  the  same 
plane ;  011  the  whole,  the  brain  of  the  Anura  is  more 
highly  organised  than  that  of  the  Urodela ;  it  is  pro- 
portionately larger  than  that  of  fishes,  but  is  still 
small.  The  brain  of  the  Anura  is  different  from 
that  of  all  other  Vertebrates,  owing  to  the  fact  that 
the  olfactory  lobes  of  the  adult  are  not  separated 
from  one  another,  and,  like  that  of  the  Urodela,  the 
cerebellum  of  the  Anura  is  of  extraordinarily  small 
size. 


422  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

In  the  Amniota  we  find  considerable  advances 
in  the  characters  of  the  brain,  which  are  chiefly  due 
to  the  angulation  of  its  several  parts,  and  the  thicken- 
ing undergone  by  the  walls  of  the  primary  vesicles  at 
various  points. 

In  the  Reptilia  the  cerebral  hemispheres  are 
always  smooth  on  their  surface,  but  they  are  now,  and 


Fig.  181.— Side  views  of  the  Brain  of  a  Tortoise  (A)  and  a  Bird  (B). 

I,  Olfactory  nerves;  \.ol,  olfactory  lobes;  VFT,  cerebral  hemispheres;  n,  optic 
nerves ;  Tro,  optic  tract ;  inf,  infundibuluin ;  H,  hypophysis  cerebri  ;  T, 
temporal  lobe;  MH,  optic  lobes;  HH,  cerebellum;  NH,  medulla  oblougata  ; 
R,  spinal  cord.  (After  Wiedersheim.) 

henceforward,  always  large  in  proportion  to  the  re- 
maining parts  of  the  brain ;  the  hemisphere  of  either 
side  is  united  to  its  fellow  by  a  transverse  band  of 
fibres  (commissure),  which  lies  just  in  front  of 
the  third  ventricle ;  the  optic  thalami  are  similarly 
united  by  a  transverse  commissure  j  the  cerebellum 


chap,  xii.]  BRAIN  OF  BIRDS.  423 

is  not  always  a  narrow  plate,  and  in  the  Crocodilia 
the  central  portion  forms  a  distinct  "  vermis." 

Of  the  thickenings  of  the  cerebral  walls,  the  two 
most  important  are  the  corpora  striata  in  the 
hemispheres,  and  the  restiform  bodies  in  the 
medulla  oblongata ;  the  former  are  the  ganglionic 
masses  which  become  developed  on  the  floor  of  the 


01 


Fig.  182  A.— Lateral  view  of  the  Brain  of  Eabbit,  to  show  the  large 
olfactory  lobes,  and  the  termination  of  the  hemispheres  in  front  of 
the  Cerebellum.  (After  Huxley.) 

A,  Frontal  lobes  ;  B,  occipital  lobes  ;  sy,  sylvian  fissure. 

brain,  and,  as  they  extend  inwards,  they  encroach  on 
the  cavity  of  the  lateral  ventricle  ;  as  may  be  sup- 
posed, they  are  largest  in  the  Crocodilia.  The  cor- 
pora restiformia  in  a  similar  manner  encroach  on 
the  fourth  ventricle. 

In  Birds  the  cerebral  hemispheres  are  propor- 
tionately still  larger  in  size,  and,  as  the  optic  lobes, 
or  so-called  corpora  bigemina  (Fig.  181  (B)  ;  MH)  are 
now  £°it  at  the  sides  and  base  of  the  brain,  the 
cerebral  hemispheres  so  overlap  them  as  to  hide 
them  when  looked  for  from  above.  The  cerebellum 
(Fig.  181  (B)  ;  HH)  is  much  larger,  and  its  lateral  lobes, 
or  flocculi,  may  be  distinguished  from  its  central 
body,  or  vermis;  while  in  section  this  division  of 
the  brain  presents  just  the  same  appearance  as  the 


424  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


OL 


S'Jl 


I.0o. 


01 


Fig.  182.— Lateral  views  of  the  Brain  of  :  B,  A  Pig.  c,  A  Chimpanzee, 
drawn  of  nearly  the  same  absolute  size. 

Ol,  Olfactory ;  A,  frontal;  B,  occipital :  c,  temporal  lobes;  s.?/,  sylvian  fissure; 
in,  island  of  Reil ;  s.o?%  supraorbital ;  s  F,  Btiperfor :  M  F,  middle ;  I  F,  inferior 
frontal  gyri ;  A  P,  antor>>-parietal ;  p  p,  postero-parietal  gyri ;  u,  ml  CM  of 
Rolando;  p  Pi,  postero-parietal  lol>ule;  opf,  occipito-temporal  sulcus;  MI, 
angular  gyms  :  2,  3,  4,  annectent  gyri ;  A  T,  M  T,  p  T,  temporal ;  s  oc,  11  oc, 
ioc.  occipital  gyri.  (After  Huxley.) 


Chap.  XII.] 


BRAIN  OF  MAMMALS. 


425 


so-called  "arbor  vitse"  of  the  human  brain,  it  is 
marked  externally  by  fairly  deep  transverse  fissures. 
The  external  surface  of  the  cerebral  hemispheres  is 
smooth,  but  the  corporate  striata  are  very  well  de- 
veloped. 

The  most  important  and  instructive  changes  are 
to  be  seen  in  the  brain  of 
the  Mammalia  ;  these 
depend  chieflyon the  great 
development  of  the  com- 
missures, which  connect 
the  two  halves  of  the 
brain  with  one  another, 
and  on  the  gradually  in- 
creasing sizeof  the  cerebral 
hemispheres  which  ends 
in  their  having  an  extra- 
ordinary predominance 
over  the  other  parts  of 
the  brain  ;  hand  in  hand 
with  their  increase  in 
size  and  extent  is  the 
improvement  of  the  in- 
tellectual faculties.  But 
the  cerebral  hemispheres 
do  not  merely  increase 
in  bulk,  their  surface  be- 
comes marked  by  grooves.  Figvl83TBrai™f  rupai>  t°  sh°w 

,      ,  J  ^   n  the  large  Olfactory  Lobe,   the 

and  the  amount  of  sur- 
face thereby  developed  is, 
as  we  have  already  said, 
greatly  extended  without 

any  corresponding  or  proportionate  increase  in  the 
size  of  the  cranial  cavity. 

The  olfactory  lobes  lie  more  or  less  below  the 
cerebral  hemispheres,  and  diminish  in  proportionate 
size  as  we  ascend  the  series  ;  the  cerebral  hemispheres 


ungrooved  Cerebral  Hemi- 
spheres, and  the  large  Cere- 
bellum. (After  Garrod.  P.Z.8., 
1879,  p.  304.) 


426  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

more  arid  more  extend  backwards,  and  at  last  com- 
pletely overlie  the  cerebellum.  As  they  increase  in 
size  they  become  broken  up  into  distinct  lobes, 
frontal,  occipital,  and  temporal.  The  cerebellum 
diminishes  in  proportionate  size,  and  the  flocculi 
cease  to  be  conspicuous  at  its  sides.  (Compare  ol,  in 
Fig.  182;  A,  B,  and  c.) 

Transverse  commissures  are  always  richly  deve- 
loped, the  corpus  callosum  connecting  the  two 
cerebral  hemispheres,  and  the  pons  varolii,  which 
bridges  over  the  hind-brain,  being  parts  which  are 
developed  in  mammals  only.  The  optic  lobes  are 
divided  transversely,  so  that  the  "  corpora  bigemina  " 
of  the  lower  vertebrates  are  now  the  "  corpora  quacl- 
rigemma  "  ;  this  mid-brain  is  proportionately  small. 

A  very  complete  series  of  gradations  of  all  these 
differential  characters  is  to  be  observed  as  we  pass  up 
the  scale  of  the  Mammalia.  This  is  to  be  seen,  first 
of  all,  in  the  proportionate  increase  in  the  weight  of 
the  brain,  as  compared  with  the  rest  of  the  body,  for, 
while  that  of  the  rabbit  is  about  y^th  part,  that  of 
man  is  ^th. 

In  the  Prototheria  the  corpus  callosum  is  always 
small,  and  the  cerebral  hemispheres,  which  are  smooth, 
do  not  cover  the  cerebellum.  The  Metatheria 
differ  a  good  deal  among  themselves.  Among  the 
Euttieria,  the  Insectivora  exhibit  a  brain  of  very 
low  character;  the  cerebral  hemispheres  are  often 
quite  smooth,  the  olfactory  lobes  are  large,  and  project 
in  front  of  the  hemispheres,  which  only  just,  if  at 
all,  overlap  the  cerebellum  behind  (as  in  Tupaia; 
Fig.  183).  This  latter  has  a  large  vermis.  The 
corpus  callosum  is  thin  and  nearly  straight,  while 
the  corpora  quadrigemina  are  proportionately  large. 
The  pons  varolii  is  very  small.  In  the  hedgehog 
there  is  a  single  simple  groove  (sulcus)  on  either 
hemisphere. 


Chap.  XII.l 


BRAIN  OP  MAMMALS. 


427 


There  early  ap- 
pears a  fissure  at 
the  side  of  the 
cerebral  hemi- 
spheres, the  sylvian 
fissure  (Figs.  182; 
sy;  and  184  (u) ;  s), 
which  separates  the 
frontal  from  the  oc- 
cipital lobe ;  this, 
which  is  very  shal- 
low in  the  rabbit  or 
the  musk-deer  (Fig. 
184;  s),  is  deeper 
in  the  pig  or  the 
dog,  and  in  man 
divides  into  an  an- 
terior and  a  pos- 
terior groove,  be- 
tween which  is 
placed  the  island 
of  Beil.  The  sur- 
face of  the  hemi- 
spheres is  next 


8,  Pi 


Fig.  184.— Brain  of  Musk-Deer.    A,  from  the  side  ;  B,  from  above. 


(•sure  of  Sylvius ;  ss,  superior  external  gyrus ;  m,  middle  ;  ii,  inferior  external 
1S75US  '  1^lppocaml)al  syrus  :  o,  supraort-ital  gyrus.    (.After  Flower,  P.Z.S., 


428  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

broken  up  into  simple  folds  or  gyri,  by  the  forma- 
tion of  intervening  fissures  ;  the  arrangement  of 
these  is  better  studied  in  a  small  than  in  a  large 
animal,  for  with  increase  in  size  the  primitive  pattern 
is  obscured  by  the  increase  of  the  convolutions 
(Flower).  These  gyri  may  be  distinguished  as  the 
superior,  middle,  and  inferior  external  gyri  (Fig.  1 84  ; 
8,  m,  i) ;  below  is  the  temporal  lobe,  separated  by  the 
hippocampal  sulcus  (A).  To  these  other  grooves  may 
be  added  on,  such  as  the  supraorbital  (Fig.  184;  o), 
and  the  complexity  of  the  surface  of  the  brain  be 
increased  by  the  development  of  annectent  gyri 
between  the  primary  folds  of  a  simpler  brain. 

It  is  not,  however,  to  the  surface  that  the  complexi- 
ties of  the  brains  of  the  higher  Mammals  are  limited, 
the  inner  as  well  as  the  outer  face  of  the  cerebral 
hemispheres  becomes  convoluted.  The  corpus  callosum, 
which  is  at  first  a  thin  straight  band  of  connecting 
fibres,  becomes  thicker,  especially  in  front  and  behind, 
and  so  curved  on  itself  that  anteriorly  it  forms  the 
"  genu  "  of  human  anatomy.  Behind  and  below  this 
corpus  callosum  is  the  "  fornix,"  and  these  two 
structures  are  peculiar  to  mammalian  brains ;  the 
former  is  developed  from  what  is,  morphologically,  the 
inner  portion  of  the  surface  of  each  cerebral  hemisphere, 
and  there  is,  therefore,  a  space  left  which  is  bounded 
on  either  side  by  a  thin  wall  (septum  lucidum)  ;  this 
space  is  known  as  the  fifth  ventricle,  but  the  name 
is  an  unfortunate  one,  inasmuch  as  this  fifth  ventricle 
is  not  developed,  as  are  the  others,  from  the  original 
cavity  of  the  cerebrospinal  axis,  but  is  merely  a  space 
between  two  overgrown  walls.  The  fornix  is  similarly 
derived  from  the  hinder  part  of  the  walls  of  the  cere- 
bral hemispheres. 

The  thickening  in  the  floor  of  the  cerebral  hemi- 
sphere of  either  side  (corpus  striafuiii)  is  much 
more  prominent  in  the  Mammalia  than  in  other 


chap  xii.]  BRAIN  OF  MAN.  429 

Vertebrates  ;  behind  this  is  a  less  conspicuous  thick- 
ening (the  hippocampus  major,  to  which  is 
added  on  in  the  higher  Primates  the  hippocampus 
minor. 

The  average  weight  of  the  human  brain  is, 
for  males,  between  46  and  53  oz.,  and  for  females 
between  41  and  47  oz.,  but  the  range  of  difference  is 
much  greater  than  this.  As  is  well  known,  the  brain 
of  Cuvier  weighed  64  oz.,  or  4  Ibs.,  while  that  of  an 
anonymous  sane  man  was  only  34  oz.,  or  but  little 
more  than  half  that  of  the  great  anatomist ;  but  the 
weight  only  must  not  be  taken  into  consideration ;  the 
depth  and  extent  of  the  convolutions  must  also  be 
estimated,  and  Wagner  has  found  a  difference  of  as 
much  as  15  per  cent,  in  the  extent  of  the  surface  of 
the  cerebral  hemispheres  of  two  selected  n  ales.  But 
that  this,  again,  is  not  all  is  not  only  clear  from  the 
consideration  that  a  small  well-made  watch  often 
keeps  better  time  than  a  kitchen  clock,  but  by  the 
following  facts  : 

(1)  The  anterior  portion  of  the  cerebrum  is  fed  by 
the  carotid  and  the  hinder  by  the  vertebral  arteries ; 
as    the    former   are    much    larger   than  the  latter,  it 
follows  that  the  anterior  portion  of  the  brain  is  better 
supplied   than  the   posterior,  and  that  pro  tanto  the 
advantage  lies  not  in  the  greater  size  of  the  cerebral 
hemispheres  as  a  whole,  but  in  the  size  of  the  anterior 
portion,  or  that  which  lies  in  front  of  the  ear. 

(2)  Though  absolutely  the  human  brain  is,  on  the 
average,  heavier  than  that  of  all  mammals  except  of 
the  elephant,  wrhich  weighs  between  8  and  10  Ibs.,  or 
of  some  whales,  which  may  weigh  as  much  as  5,  while 
the  horse,  for  example,  has  a   brain   weighing  only 
23  oz.,  and  an   average-sized  dog  less  than  7  oz.,  yet, 
in  the  apparently  more  important  relation  of  brain 
weight  to  body  weight,  in  which  man  presents  the 
proportions  of  T^,  he  is  surpassed  by  some  American 


43°  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

a,pes,  in  which  it  varies  from  -i-  to  T\,  by  the  sparrow 
in  which  it  is  ^T,  and  the  titmouse  in  which  it  is  j1^ 
(Bischoff). 

On  the  other  hand,  when  we  compare  man  with 
his  nearest  zoological  allies,  we  find  that  not  only  is 
the  capacity  of  his  skull  and  the  weight  of  his  brain 
greater,  but  that  there  is  a  notable  increase  in  the 
complexity  of  the  secondary  gyri  of  the  surface  of 
his  cerebral  hemispheres,  as  compared  with  those 
of  the  apes. 

The  spinal  cord  differs  from  its  anterior  enlarge- 
ment, the  brain,  in  having  the  grey  ganglionic  mate- 
rial placed  internally  to  the  white  fibrous  cords,  which 
act  as  the  conductors  of  nervous  stimuli,  but,  like  it, 
it  is  hollow  internally,  and  the  epithelium  whicli 
lines  it  is  temporarily  or  permanently  ciliated. 
It  is  marked  above  and  below  by  a  median  groove, 
and,  in  all  vertebrates,  has  paired  nerves  issuing  from 
it,  each  of  which  is  connected  with  it  by  a  superior 
and  an  inferior  root.  It  is  cylindrical  in  all  Verte- 
brates except  the  Cyclostomata  and  Chimsera  ;  not 
unfrequently  it  extends  throughout  the  whole  length 
of  the  neural  canal  formed  by  the  spinal  column,  but 
in  the  sun-fish  it  is  greatly  shortened,  so  as  to  look 
indeed  like  a  mere  appendage  to  the  brain,  and  in  the 
anurous  Amphibia,  in  Birds,  and  various  Mammals 
(among  which  are  the  hedgehog  and  man),  the 
terminal  portion  is  filamentous,  and  is  accompanied  on 
either  side  by  a  number  of  nerves,  thereby  giving  rise 
to  the  so-called  cauda  equiiia  (horse's  tail). 

SENSORY    ORGANS. 

It  has  been  already  stated  that  all  the  organs 
of  sense  have  their  primitive  seat  in  that  outer 
layer  of  the  body  which,  in  the  embryo,  is  called  the 
epiblast  or  ectoderm ;  and  we  have  already  learnt 
that  the  nervous  system  itself  does,  in  most  cases, 


Chap.  XII.] 


SEWSOX  Y  '  ORGANS. 


remain  throughout  the  life  of  the  animal  in  close  local 
contact  with  the  outer  world.  In  tracing  the  history 
of  the  organs  of  sense  we  shall  find  that,  whatever 
their  final  position,  they  too  are  essentially  of  epiblastic 
origin.* 

Among  the  Hydroid  polyps,  where  no  nervous 
system  has  as  yet  been  made  out,  we  observe  that  the 
tentacles  which  surround  the  mouth  are  provided  with 
fine  hair-like  projections,  which  look  not  unlike  a 
trigger ;  these  processes  are  seen  to  be  in  connection 
with  cells  which  differ  in  character  from  their  neigh- 
bours by  the  possession  of  a  coiled  up  thread;  when 

*  Since  the  above  was  put  into  type,  Prof.  Charles  Stewart 
has  favoured  me  with  an  account  of  his  observations  on  sense  cells 
in  sponges,  and  with  the  accompanying  illustrative  figure  (Fig. 
184  A).  It  is  found  that 
"  the  external  orifices 
of  the  interradial  canals 
of  Grantia  compressa 
are  fringed  with  deli- 
cate hair-like  processes 
of  the  soft  substance  of 
the  sponge.  At  first 
sight  these  remind  one 
of  the  palpocils  of  Hy- 
dra, which  they  closely 
resemble  in  general 
form  and  size  "  ;  from 
these,  however,  they 
differ  in  important  par- 
ticulars. The  processes 
or  hairs  vary  in  length 
from  nfotfth  to  about  Fig.  184  A. 

ysV&th    of      an    inch ; 

their  base  is  from  ^^Wtt1  to  T-jrtar&'th  of  an  inch,  and  they  taper 
to  a  fine  point.  All  such  as  can  be  well  seen  are  found  to  have 
a  special  relation  to  a  subjacent  branched  cell ;  this  latter  sends 
outwards  a  delicate  filament  which  traverses  the  axis  of  the  pro- 
cess. "  Such  an  apparatus  appears  both  by  position  and  structure 
to  be  specially  impressed  by  varying  conditions  in  the  inrushing 
water,  particles  in  solution  or  suspension  in  this  water  inducing 
molecular  changes  in  the  cell  at  the  base  of  the  process,  and  per- 
haps leading  to  the  contraction  of  neighbouring  cells-  In  other 
words,  these  processes  seem  to  act  as  part  of  an  automatic 
mechanism  for  regulating  the  water-currents  of  the  organism." 


432  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  tentacle  is  stimulated  we  observe  that  these 
threads  are  expelled,  and  that  they  are  barbed  ;  it  will 
be  within  the  knowledge  of  most  of  us  that  these 
thread  cells,  as  found  in  jelly-fishes,  are  efficient  organs 
of  offence.  Their  relation  to  the  trigger-like  process 
suggests  that  these  projections  are  the  first  to  feel  the 
pressure  of  any  foreign  body,  and  that  the  pressure 
communicated  by  them  to  the  thread-cell  or  nemato- 
cyst,  results  in  the  projection  of  the  contained  thread. 
Here,  then,  we  seem  to  have 
the  earliest  and  simplest 
kind  of  automatic  tactile 
or  ;;aii  including,  of 
course,  in  the  term  touch 
the  general  sensation  of 
pressure  from  without.  It 
is,  at  the  same  time,  neces- 
sary to  observe  that,  al- 
though  these  trigger-hairs 

aPPear  to  be  the  simPlest 

Schultze.)  sense  organs  of  a  multicel- 

lular  or  metazoic  animal,  yet 

that  some  of  the  unicellular  Protozoa  are  not  without 
organs  of  offence  that  are  physiologically  comparable  to 
the  threads  of  thread  cells,  for,  if  we  add  a  drop  of  iodine 
to  the  water  in  which  a  Paramo3cium  is  swimming,  we 
find  that  it  immediately  thrusts  out  from  its  body  fine 
stiff  processes.  If,  then,  these  are  comparable  to  the 
threads  of  a  hydroid,  it  is  clear  that,  functionally  also, 
the  ectosarc  of  an  infusorian  is  comparable  to  the 
sensory  parts  of  the  epithelium  of  a  hydroid,  and  is, 
like  it,  capable  of  responding  to  definite  external 
stimuli  in  a  definite  way.  It  is  important  to  observe 
that  the  first  indication  of  tactile  organs  is  associated 
with  the  protection  of  the  individual,  as  much  as  with 
the  function  of  paralysing  the  prey  which  is  seized 
upon  for  food.  As  the  sensory  cells  remain  superficial 


Chap,  xii.]  TACTILE  ORGANS.  433 

in  position  in  the  Cceleiiterata,  the  absence  of  special 
tactile  organs  in  most  of  the  members  of  the  group  is 
not  to  be  wondered  at,  for  the  tentacles,  as  a  whole, 
may  be  looked  upon  as  having  a  general  tactile  sense. 

Among  the  Turtoellaria,  trigger-hairs  in  con- 
nection with  nematocysts  have  been  observed ;  in 
many  cases  tufts  of  delicate  hairs  have  been  found 
scattered  over  the  whole  body,  but  more  especially 
well  developed  at  its  sides.  In  some  there  are 
definite  tactile  organs  in  the  shape  of  tentacles,  which 
are  best  developed  in  the  anterior  regions  of  the  body, 
and  on  which  the  sensory  hairs  are  particularly  nume- 
rous. Thysanozoon,  which  is  remarkable  for  having 
the  dorsal  surface  covered  with  villiform  projections 
of  the  body  wall,  has  a  bundle  of  such  hairs  at  the  tip 
of  each  villus.  In  the  earthworm,  the  whole  body  of 
which  is  very  sensitive  to  tactile  impressions,  the 
anterior  end  is  most  remarkably  so  ;  in  the  polychsetous 
Annelids  specially  modified  sense-cells  are  largely 
developed  on  the  protruding  antennae  and  tentacles 
which  are  developed  on  the  praBstomium,  and  are 
supplied  by  nerves  which  arise  directly  from  the 
cerebral  ganglia ;  these,  as  well  as  those  on  other  parts 
of  the  body,  are,  like  the  antennas  of  the  Arthropoda 
and  of  some  Mollusca,  very  important  aids  to  the 
organism,  for  they  are  capable  of  movement  laterally, 
or  of  protrusion  forwards,  or  of  both ;  they  are,  in 
other  words,  able  to  feel  about,  and  not,  as  is  the  case 
with  the  earthworm,  compelled  to  wait  for  the  arrival 
of  food  or  foe. 

In  the  Hirudinea  the  widely  distributed  organs 
of  general  tactile  sense  are  purely  of  epidermic  origin, 
and  are  known  to  be  supplied  with  nerve  fibres ;  at 
the  anterior  end  of  the  body  these  cells  are  aggregated 
to  form  the  so-called  goblet-shaped  organs.  According 
to  Whitman,  special  papilliform  aggregations  are  to 
be  found  on  every  segment  of  the  body. 
cc— 16 


434  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

When  the  outer  surface  of  the  body  becomes 
hardened  by  the  dejposit  in  its  wall  of  chitin,  as  in  the 
Arthropoda,  or  of  calcareous  salts,  as  in  the  Echino- 
dermata,  or  by  the  development  of  a  shell,  as  in  the 
Mollusca,  the  general  tactile  sense  becomes  more  dis- 
tinctly limited  ;  this  is,  perhaps,  least  noticeable  in  the 
Ediinodcrmata,  where  the  superficial  plexus  of 
nerve  filaments  extends  over  the  test  and  along  the 
projecting  suckers,  while  special  nerve  cells  are  deve- 
loped in  the  highly  sensitive  pedicellariae. 

In  the  Arthropoda  the  special  tactile  organs  are 
seen  at  their  simplest  in  Peripatus ;  in  it  the  dorsal 
surface  is  raised  up  into  delicate  imbricated  papillae, 
from  the  tip  of  which  there  projects  a  fine  process.  In 
others  they  take  on  the  forms  of  projecting  rods.  As 
we  all  know,  we  have  only  to  stroke  lightly  the  hairs 
on  our  own  arms  to  discover  how  easily  tactile  sensa- 
tions are  conveyed  by  more  or  less  stiff  processes  to 
the  sensory  cells  that  lie  at  their  base.  Where  the 
greater  part  of  the  integument  is  hardened  it  is  clear 
that  projecting  rods  or  "hairs"  will,  if  they  be 
provided  with  nerve  fibres,  and  continuous  with 
sensory  cells,  convey  to  the  underlying  and  protected 
nervous  system  any  movement  of  their  free  ends ;  the 
movement,  then,  of  these  hairs  becomes  in  an  Arthro- 
pod a  sense  of  touch  ;  these  rods  are  not  confined  to 
the  antennae,  for  they  are  developed  on  very  various 
parts  of  the  bodies  of  Arthropods. 

Sagitta,  in  which  there  is  likewise  a  chitinous 
cuticle  investing  the  body,  has  a  large  number  of 
bundles  of  stiff  setae  scattered  over  the  surface  of  its 
integument  (Fig.  186). 

Among  the  Chordata  we  find  that  little  is 
definitely  known  as  to  the  tactile  organs  of  the  two 
lower  groups  ;  the  only  sensory  cells  that  have  as  yet 
been  recognised  in  Amphioxus  are  of  a  much  sim- 
pler character  than  those  which  we  have  just  been 


Chap.  XII.] 


ORGANS  OF  TASTE. 


435 


considering  ;  these,  which  are  most  numerous  on  the 
cirri  and  in  the  neighbourhood  of  the  mouth,  lie  side 
by  side  with  the  ordinary  epithelial  cells,  from  which 
they  are  to  be  distinguished  by  a  stiff  free  process,  and 
a  basal  connection  with  a  nerve  fibre,  calling  to  mind 
again  the  simple  sense  cells  of  the  Medusae.  A  very 
ordinary  character  of  tactile  cells  among  the 
Verteforata  is  their  bulb-like  arrangement  (see 
"  Elements  of 
Histology,"  chap, 
xv.) ;  they  are, 
as  may  be  sup- 
posed, widely  dis- 
tributed over  the 
whole  body,  al- 
though, of  course, 
they  are  much 
more  richly  de- 
veloped in  some 


Fig.  186.— Tactile  Organ  of  Sagitta  bipunctata, 
showing  the  long  stiff  setae.  (After  O. 
Hertwig.) 


in 


parts      than 
others,     and     in 
some  forms  more 
than  in  others. 

Organs  of  taste. — Although  we  may  well 
suppose  that  some  sense  of  taste  is  possessed  by  the 
lower  Metazoa,  we  have  as  yet  very  little  definite 
information  as  to  organs  to  which  it  is  reasonable  to 
ascribe  such  a  function.  In  the  Echinoidea  (ex- 
cepting Cidaris)  Loven  has  described,  under  the  name 
of  sphseridia*  organs  to  which  he  assigns  a 
gustatory  function.  These  are  always  set  around 
and  confined  to  the  region  of  the  mouth  (actinostome), 
where  they  have  the  general  appearance  of  transparent 
solid  bodies  invested  by  pigmented  cells  and  a 
ciliated  epithelial  layer.  Just  as  the  auditory  organs 
of  some  Ccelenterates  appear  to  be  modified  tentacles, 
so  do  the  sphseridia  remind  us  in  the  most  striking 


436  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

way  of  the  structure  of  the  spines  of  Echinids.  Their 
small  size  and  protected  position,  under  large  spines 
or  in  special  cavities  of  the  test,  prevent  us  from 
regarding  them  as  tactile  organs,  while  their  constant 
approximation  to  the  entrance  into  the  digestive 
tract  justifies  us,  at  present,  in  ascribing  to  them  the 
function  of  testing  the  food  which  is  found  in  the 
water  in  which  their  possessor  lives. 

Very  little  is  definitely  known  as  to  the  organs  of 
taste  in  other  Invertebrata,  although,  of  course,  most 
do,  on  observation,  exhibit  some  kind  of  preference 
for  certain  foods  ;  this  was  seen  by  Mr.  Darwin  even 
in  the  omnivorous  earthworm.  In  Insects  the 
maxillary  palpi  are  probably  the  seat  of  the  organ, 
and  Lowne  has  described  those  of  the  blowfly  as 
having  their  cavity  filled  with  cells,  which  are  supplied 
by  a  branch  from  the  great  nerve  trunk  of  the 
proboscis.  Freely  projecting  epithelial  papillae,  not 
unlike  the  gustatory  organs  of  tadpoles,  have  been 
observed  on  the  tentacles  of  various  Molluscs ;  the 
cells  of  which  these  papillae  are  composed  are  ciliated, 
and  appear  to  be  well  supplied  with  nerves ;  their  gus- 
tatory function  seems  to  have  been  demonstrated. 

Nothing  is  certainly  known  as  to  gustatory  organs 
in  the  Urochordata  or  Cephalocfiordata.  In 
Fishes,  the  organs  of  this  sense  are  only  feebly 
developed,  and,  as  often  happens  with  organs  in  a 
generalised  condition,  they  are  not  so  definitely  localised 
as  in  the  higher  forms.  The  cup-shaped  organs  have 
at  their  edge  long  cylindrical  cells,  with  more  delicate 
cells  in  the  central  portion  ;  they  are  not  confined  to 
the  cavity  of  the  mouth,  but  are  found  also  on  the 
skin  (compare  the  account  of  the  teeth  of  Elasmo- 
branchs,  page  141);  those  that  are  placed  on  the 
mucous  membrane  of  the  palate  are  supplied  with 
branches  from  the  glosso-pharyngeal  nerve.  In  the 
carps  they  are  described  as  being  most  largely  developed 


chap,  xii.]  ORGANS  OF  TASTE.  437 

on  the  palate,  on  the  rudimentary  tongue,  on  the 
mucous  membrane  which  covers  the  inner  side  of  the 
branchial  arches,  and  the  barbels ;  around  the  mouth, 
on  the  skin  of  the  head,  and  the  rest  of  the  body  they 
are  less  numerously  developed. 

In  the  Amphibia  the  cells  of  this  sense  are 
grouped  into  discs,  the  so-called  gustatory  discs  ;  those 
on  the  tongue  are  placed  on  elongated  papillae,  but 
such  as  have  been  observed  on  the  mucous  membrane 
of  the  palate  are  not  known  to  project  above  the 
surface,  except  in  the  region  of  the  vomerine  bones, 
where,  as  on  the  tongue,  the  papillae  that  bear  them 
may  be  distinguished  as  fimgiform.  The  Amphibia 
exhibit  a  higher  form  of  differentiation  than  the 
fishes,  inasmuch  as  the  gustatory  cells  appear  to  be 
confined  to  the  region  of  the  mouth.  For  the  majority 
of  the  Saiiropsida  it  is  impossible  to  affirm  definitely 
the  possession  of  a  sense  of  taste,  and  it  is  very 
probable  that  in  many,  as  in  some  (e.g.  Birds)  almost 
certainly,  the  sensations  experienced  are  those  of  a 
foreign  body  only  ;  are,  in  fact,  mechanical,  and  not 
chemical.  In  Lizards  and  in  Crocodiles  there  are, 
however,  projections  of  the  mucous  membrane  (papillae) 
which  are  provided  with  goblet-shaped  cells,  and  these 
may,  by  analogy,  be  reasonably  supposed  to  have  a 
gustatory  function. 

Just  as  the  ant-eater,  and  other  Mammals,  prove 
to  us  that  the  tongue  may  be  a  seizing  organ,  and  is 
not  merely  the  bearer  of  the  gustatory  bulbs,  so,  in 
man  at  any  rate,  the  gustatory  function  is  not 
confined  to  the  body  of  the  tongue,  for  in  ourselves 
the  soft  and  part  of  the  hard  palate  are  also  capable  of 
taste.  The  greater  number  of  gustatory  sensations 
are,  nevertheless,  experienced  through  the  tongue, 
and  we  may  justly  say  that,  in  this  particular,  the 
fish  stands  at  one,  and  the  mammal  at  the  other  end  of 
the  series.  The  majority  of  the  gustatory  cells  are 


438  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

set  upon  papillae,  and  are  most  numerous  on  the  cir- 
cumvallate  papillae  at  the  back  of  the  tongue ;  in 
rabbits  and  hares  a  large  supply  of  taste  bulbs  is  to  be 
found  on  an  organ  developed  on  either  side  of  the 
root  of  the  tongue,  which  is  broken  up  into  ten  to 
fourteen  valleys,  in  the  recesses  of  which  the  bulbs  are 


Fig.   187.— A,  Taste  Bulbs  of  Babbit ;   B,  Transverse  Section  through 
Taste  Folds  of  Eabbit.    ( Ai ter  Engelmann. ) 

placed.  In  this  sense,  then,  as  in  others,  we  find  that 
the  terminal  sense  organs  are  withdrawn  from  the 
surface,  protected  from  rough  contact,  and  excited 
only  by  certain  definite  stimuli.  This  must  not  lead 
us  to  suppose  that  the  gustatory  sense  organs  offer 
any  exception  to  the  rule  that  all  organs  of  sense  have 
their  origin  in  the  epiblast  of  the  embryo. 

Olfactory  organs. — Till  we  reach  the  Arttiro- 
poda  and  Ulollusca  we  do  not  find  any  structures 


Chap,  xii.]  OLFACTORY  ORGANS.  439 

which  can  be  definitely  asserted  to  have  an  olfactory 
function.  In  the  higher  Crustacea  we  find  organs  in 
the  antennules  which,  in  the  crayfish,  are  thus  dis- 
posed ;  the  outer  branch  (exopodite)  has  attached  to 
the  greater  number  of  its  more  distal  joints  tufts  of 
short  delicate  bristles,  flattened  or  papilliform  at  their 
free  ends;  these  bristles  have  granular  contents,  and 
are  supplied  by  fine  nerve  fibres.  In  the  Insecta, 
where  there  is  only  one  pair  of  antennae,  the  olfactory 
organ  is,  to  judge  from  the 
accounts  of  Braxton  Hicks 
and  Lowne,  placed  in  the 
third  joint  of  the  antennae 
of  the  blowfly  ;  the  surface 
of  this  joint  is  described  as 
being  "  covered  with  minute 
hairs,  between  which  are  a 
vast  number  of  pellucid  dots, 
about  17,000  or  18,000  on 
each  antenna,  with  about  *'*&•  187  A.— olfactory  Appen- 

•    1,1  •  i  dage  of  Exopodite  of  an  tennule 

eighty  large  irregular  spots  of  Crayfish ;  x  soo.  a.  Front,  b. 
of  a  similar  character."  The  Side  View.  (After  Huxley.) 
smaller  dots  appear  to  be  the  optical  expression  of 
the  orifices  of  minute  sacculi,  and  the  larger  the 
common  openings  of  compound  sacculi.  This  third 
antennal  joint  is  described  as  being  filled  with  a  cellular 
pulp,  through  which  are  distributed  the  fibrils  of  the 
antennary  nerve. 

In  the  Mollusca  the  olfactory  organ  ("os- 
phradiiim,"  Lankester)  is  remarkable  for  its 
constant  relation  to  the  neighbourhood  of  the  respi- 
ratory orifice,  and  its  as  constant  nerve  supply  from 
the  visceral  commissures ;  it  appears  to  be  absent  in 
air-breathing  forms  (e.g.  the  snail),  and  we  may 
suppose,  therefore,  that  it  has  a  function  in  the  way  of 
testing  the  water  which  carries  the  oxygen  necessary 
for  respiration.  It  has  ordinarily  the  form  of  a  short 


44°  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

canal,  which  either  ends  blindly,  or  is  bifurcated  at  its 
free  end ;  at  this  end,  or  at  the  point  of  bifurcation, 
there  is  a  small  ganglion.  The  cylindrical  canal 
-consists  of  a  network  of  coiled  fibrous  bands,  and  is 
invested  by  elongated  epithelial  cells,  which  are 
directly  continuous  with  the  integument ;  these  cells 
ara  very  richly  supplied  with  nerve  fibres. 

Among  the  Ch  or  data  no  definite  olfactory  organ 
has  been  recognised  in  the  TJrodiordata ;  in  the 
rest  it  always  stands  in  close  relation  to  the  respiratory 
orifice,  but  in  nearly  all  fishes  it  is  not  directly 
continuous  with  the  respiratory  passages.  The  single 
pit  at  the  anterior  end  of  the  body  of  AmpMoxus, 
though  lined  with  a  ciliated  epithelium,  can  by  no 
means  be  certainly  said  to  be  an  olfactory  organ. 
The  Cyclostomata  have  but  a  single  pit,  whence  they 
have  been  distinguished  from  all  other  Vertebrata  as 
the  Monorrhina ;  notwithstanding  the  single  con- 
dition of  this  pit  the  nerve  supply  is  double,  and  we 
must  not,  therefore,  yield  to  the  temptation  to  regard 
this  condition  as  being  a  primitive  one ;  in  this,  as  in 
many  other  points,  the  existing  Cyclostomata  show 
that  they  stand  at  some  distance  from  the  primitive 
vertebrate  stock ;  their  single  nasal  pit  is,  almost 
certainly,  the  result  of  the  fusion  of  two  originally 
separate  sacs  ;  this  view  is  supported  by  the  observation 
that,  in  the  larval  lamprey,  the  sac  is  more  nearly 
divided  into  two  internally  than  it  is  in  the  adult. 
The  interior  of  the  cavity  is  occupied  by  folds,  some 
of  which  project  farther  inwards  than  others,  and  all 
of  which  are  covered  by  a  mucous  membrane  ;  to  this 
are  distributed  branches  of  the  olfactory  nerves.  In 
the  lampreys  the  sac  is  closed  posteriorly,  but  in 
Myxinoids  it  opens  into  the  cavity  of  the  mouth. 

In  all  the  rest  of  the  Vertebrata  the  olfactory 
organs  arise  from  a  pair  of  patches  of  epiblast  in  front 
of  the  mouth,  which,  as  they  thicken,  give  rise  to  a 


Chap,  xii.]  OLFACTORY  ORGANS.  441 

pit-shaped  cavity ;  the  epithelial  cells  that  line  this 
pit  are  the  end  organs  of  the  olfactory  sense,  and  the 
whole  layer  forms  the  so-called  Sdmeideriaii 
membrane,  which  gradually  becomes  more  and 
more  elaborately  folded.  The  sac  does  not  remain 
pit-like  in  fishes,  but  becomes  connected  by  a  groove 
with  the  angle  of  the  mouth  ;  this  groove,  which  may 
become  of  some  depth  (rays),  is  covered  over  by  a  fold 
of  the  integument,  the  so-called  nasal  valve  (Fig.  188)  ; 
so  that  we  are  able  to  distin- 
guish an  anterior  and  a  pos- 
terior orifice,  the  hinder  of 
which  is  in  close  relation  to, 
but  is  not  within,  the  cavity  of 
the  mouth. 

Tn  the  Dipnoi  the  hinder 
orifices  come  to  lie  within  the 

buccal  area,   and  the  same  is      Fig  m_Nasal  Groove 
true  or  all  the  pentadactyle  V  er-  of  the  Dog-fish, 

tebrata,  in  which,  as  we  ascend  «*  1Sg^ffyUfS\  r\ 
the  series,  we  find  the  posterior  S!froove-  (AfterGegen- 
nares  coming  to  lie  farther  and 

farther  back,  as  the  various  bones  of  the  roof  of  the 
mouth  form  outgrowths  which  serve  as  a  floor  for  the 
nasal  passages.  We  cannot  resist  the  supposition  that 
this  movement  in  the  position  of  the  posterior  nares 
is  in  relation,  firstly,  to  the  altered  mode  of  respiration, 
the  lungs  taking  the  place  of  the  gills ;  and,  secondly, 
to  the  needs  of  the  organism.  If  we  may  judge  from 
the  crocodile  or  the  whale  (page  242),  the  elongated 
passage  has  not  essentially  any  relation  to  the  olfactory 
sense ;  the  true  olfactory  portion  remains  throughout 
the  Vertebrata  a  closed  pit,  and  the  only  advantage  to 
it  that  results  from  the  elongation  of  the  passage  is  a 
mechanical  one.  The  longer  air  passages  allow  of  a 
more  forcible  inspiration,  and,  in  consequence,  of  a 
more  forcible  taking  in  of  odoriferous  particles. 


442  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  nasal  sacs,  then,  of  different  Vertebrates,  differ 
only  in  the  extent  of  the  complication  of  their  internal 
walls,  and  of  the  membrane  which  covers  them.  This 
complication  is  chiefly  effected  by  the  development  of 


Fig.  189. — Longitudinal  Section  through  a  Dog's  Nose,  showing  the 
Spongy  Bones. 

o,  Region  of  the  olfactory  sense  ;  6,  air  chamber  ("  sneezing  region  ") ;  c,  a  bristle 
passed  thi'ough  the  nostril  into  the  nasal  chamber;  d,  a  bristle  from  the 
nasal  chamber  into  the  passage  by  which  the  latter  communicates  with  the 
mouth.  (After  T.  J.  Parker.) 

cartilages,  which  may  become  more  or  less  completely 
ossified,  in  the  upper  or  olfactory  region ;  these  arise 
from  the  side  walls  of  the  cavity,  and  project  into  it; 
such  bones  are  known  as  the  turMnate  bones. 
While  Reptiles  and  Birds  have  one  only,  Mammals 
have  three  (excepting  the  Cetacea,  which  have  none)  ; 
these  vary  greatly  in  form  and  in  the  extent  to  which 
they  are  developed,  and,  as  they  are  covered  by  the 


chap,  xii.]       SMELLING  AND  SCENTING.  443 

olfactory  membrane,  we  may  estimate  the  comparative 
complexity  of  the  turbinate  bones  by  the  acuteness  of 
smell  of  their  possessor.  Many  mammals,  both  those 
that  hunt  (Felidse),  and  those  that  are  hunted  (Cer- 
vidre),  have  a  much  more  acute  sense,  and  more  com- 
plicated turbinate  bones,  than  has  man  (Fig.  189). 

Like  other  specialised  sensory  organs,  the  olfactory 
apparatus  of  Vertebrates  is  provided  with  character- 
istic cells,  which  are  to  be  found  in  the  lamprey  almost 
as  well  marked  as  in  man.  (See  "  Elements  of  His- 
tology," Fig.  166.) 

In  the  physiology  of  this  sense  it  is  necessary  to 
distinguish  between  smelling;,  which  is  a  more  or 
less  passive  act,  and  scenting1,  which  is  an  active 
operation.  Although  we  cannot  suppose  that  the 
latter  power  is  well  developed  among  Fishes,  yet  the 
fact  that  the  nasal  valve  is  provided  with  muscles, 
taken  in  connection  with  what  we  know  as  to  the 
habits  of  sharks,  for  example,  justifies  in  believing  that 
some  fishes,  at  any  rate,  are  capable  of  scenting  as  well 
as  of  smelling.  In  the  Sauropsida  a  more  forcible  in- 
spiration of  air  must  be  the  chief  aid,  but  in  Mammals 
the  addition  of  external  movable  cartilages  supplied 
with  muscles  results  in  a  power  to  enlarge  or  diminish 
at  will  the  size  of  the  entrance  to  the  nasal  passages. 

The  external  cartilaginous  "nose"  once  formed  may 
become  adapted  to  duties  altogether  foreign  to  the 
olfactory  sense  ;  it  may  be  prolonged  into  a  snout 
which,  as  in  the  pig,  may  be  of  real  use  as  a  digging 
organ,  or  it  may  become,  as  in  the  elephant,  greatly 
elongated,  and  have  the  functions  of  a  prehensile  trunk, 
or  proboscis. 

The  sense  of  sight  is  at  first  a  generalised  property, 
many  Protozoa  showing  themselves  to  be  sensitive  to 
light.  The  most  primitive  condition  of  an  eye  or  optic 
organ  is  presented  by  patches  of  pigment  which  are 
more  sensitive  to  light  than  is  protoplasm  generally. 


444  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Such  eye- spots  are  possessed  by  a  number  of  the 
lower  Invertebrata.  They  are,  perhaps,  found  at  their 
simplest  condition  in  a  starfish,  where  they  occupy  the 
ends  of  the  arms  ;  and  these  ends  are,  by  a  muscle  run- 
ning along  the  upper  surface  of  the  arm,  turned  upwards 
so  as  to  be  exposed  to  the  light.  There  are  here  (Fig. 
190)  a  number  of  eye-spots,  each  of  which  is  made 
up  of  several  epithelial  cells  j  these  undergo  a 


Fig.  190. — Four  separate  Eye-spots  of  a  Starfish,  showing  the  invaginated 
Epithelial  Cells  and  the  Central  Cavity ;  below  is  the  plexus  of 
Nerve  Fibres.  (After  Hamann.) 

certain  amount  of  invagination,  and  give  rise  to  a 
central  cavity  in  their  midst ;  with  these  cells  nerve 
fibres  become  connected,  and  in  their  substance  a  red 
pigment  is  deposited.  Here,  then,  we  have  nothing 
more  than  a  number  of  epithelial  sensory  cells,  distin- 
guished by  the  possession  of  pigment ;  the  cuticle,  it 
will  be  observed,  is  not  thickened  into  a  convex  cornea, 
and  there  is  no  reason  to  suppose  that  the  fluid  in  the 
central  cavity  has  any  refractive  action  on  the  rays  of 
light. 

The  Medusae,  or  such  of  them  as  have  eyes 
present  us  with  a  condition  which  is  a  little  in  ad- 
vance of  what  obtains  in  the  starfish ;  for,  speaking 


chap,  xii.]  OPTIC  ORGANS.  445 

generally,  we  observe  a  distinction  between  the  pig- 
mented  and  the  sensory  cells ;  the  latter  are  not  mere 
cylindrical  bodies,  but  have  their  peripheral  portion 
converted  into  an  elongated  process,  not  unlike  a  small 
rod,  while  they  are  continuous  behind  with  ganglionic 
cells.  In  the  simplest  cases  there  is  no  cornea  or 
lens,  or  organ  to  concentrate  the  rays  of  light  \  in  the 
more  complicated  the  investing  cuticle  becomes  convex 
in  shape,  and  has,  no  doubt,  some  such  function ;  so 
that  we  have  now  to  observe  an  apparatus  which  is 
composed  of  parts  that  are  respectively  refractive, 
light-absorbing,  and  light-perceiving.  These  eyes  lie 
at  the  base  of  the  tentacles,  and  have  been  proved  by 
direct  experiment  to  be  really  sensitive  to  luminous 
impressions  ;  specimens  of  Aurelia  (the  common  jelly- 
fish), which,  when  uninjured,  were  found  to  swim 
towards  a  beam  of  light  flashed  upon  the  water  in 
which  they  were  kept,  were,  when  the  eye  spots  were 
removed,  observed  to  exhibit  no  change  of  manner  oil 
the  application  of  a  similar  stimulus. 

The  earthworm  is  without  any  organs  that  can  bo 
called  eyes,  and,  as  a  general  rule,  we  find  that  bur- 
rowing forms  are  always  less  well  provided  with  optic 
organs  than  their  allies  which  live  on  the  surface  of 
the  land ;  at  the  same  time  the  worm  is  sensitive  to 
light,  and  ordinarily  withdraws  from  it ;  the  sensitive- 
ness is  confined  to  the  anterior  region  of  the  body. 
This  cannot  but  be  regarded  as  a  very  striking  phe- 
nomenon, when  correlated  with  the  concentrated  con- 
dition of  their  nervous  system,  and  the  fact  that  in 
Vermes  with  a  more  diffused  arrangement  of  the 
nervous  system,  eyes  are  found  in  various  regions  of 
the  body. 

In  the  lower  worms,  simple  eye-spots  are  not  un- 
frequently  present,  and,  as  often  happens  with  organs 
in  a  simple  or  indifferent  condition,  they  are  present 
in  large  numbers ;  some  Tiirtoellaria,  for  example, 


446  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

have  several  hundreds  ;  they  are,  as  a  rule,  best  de- 
veloped in  the  region  of  the  cerebral  ganglia,  and,  in 
some  cases,  even  in  these  low  forms,  they  are  found 
on  the  tentacles  ;  pigment  cells  are  here  also  separate 
from  sensory  cells,  and  the  latter  are  continued  into 
nervous  filaments,  which  pass  to  the  optic  nerve. 
They  are  turned  towards  all  directions,  but  exhibit  an 
advance  in  differentiation  by  lying  below  the  epithe- 
lium which  invests  the  body.  Pigment  spots  are  not 
confined  to  the  adult  forms,  the  larva  of  the  liver- 
fluke,  for  example,  having  on  its  back  two  curved 
patches,  the  convex  sides  of  which  are  opposed  to  and 
placed  close  to  one  another. 

In  higher  groups,  the  number  of  eyes  ordinarily  be- 
comes reduced,  but  even  among  the  Polychaetous 
Annelids  we  find  a  form  (Polyophthalmus)  in  which 
a  pair  of  eyes  is  developed  on  every  segment,  in  ad- 
dition to  those  on  the  head.  This  fact,  especially  when 
taken  into  consideration  with  the  presence  of  eyes  in 
the  last  segment  of  the  body  in  Fabricia  and  some 
other  worms,  is  very  significant,  as  showing  us  that 
sensory  organs,  which  are  essentially  of  epiblastic 
origin,  may  be  developed  and  retained  on  any  part  of 
the  body  in  which  their  presence  is  useful  to  their 
possessor. 

When  the  eyes  become  reduced  in  number,  there 
may  be  several  pairs  in  the  more  anterior  region  of 
the  body,  as  in  the  leech,  which  has  ten  pairs ;  or  they 
may  be  found  on  the  tentacles,  as  in  Branchiomma,  or 
on  the  gills,  as  in  Sabella.  The  next  step  in  the  re- 
duction is  seen  in  the  scorpion  and  other  Arthropods, 
where  there  are  a  pair  of  "  compound "  and  several 
pairs  of  "  simple  "  eyes  \  and  the  final  step  is  reached 
in  the  higher  members  of  all  groups,  where  the  eyes 
are  two  in  number  only ;  in  various  Entomostraca 
(e.g.  Leptodora)  the  two  eyes  become  fused  in  the 
adult. 


chap,  xii.]  OPTIC  ORGANS.  447 

The  simplest  condition  of  the  final  stage  is  to  be 
found  in  the  Nautilus,  where  the  eyes  retain  the  primi- 
tive condition  of  having  their  central  cavity  open  to 
the  exterior  ;  the  cells  which  line  this  cavity,  and 
which  are  the  direct  continuation  of  the  epithelial 
cells  which  invest  the  body,  are  converted  into  sensory 
(retinal)  cells,  and  are  connected  by  nerve  filaments 
with  the  optic  nerve  which  is  given  off  from  the  cere- 
bral ganglion.  A.  higher  stage  than  this  is  to  be  seen 
in  the  snail,  for  here  the  cup  becomes  closed  up,  and 
there  is  developed  in  its  cavity  a  spherical  body  which 
has  the  function  of  a  lens,  while  the  outer  wall  of  the 
cavity  plays  also  a  part  in  refracting  the  rays  of  light, 
owing  to  its  having  been  converted  into  a  cornea. 
Peripatus  has  an  eye  which  does  not  essentially  differ 
from  that  of  the  gastropodous  Mollusca. 

The  typical  eye  of  a  well-developed  Polychaetous 
Annelid  presents  an  advance  upon  those  of  the  just' 
mentioned  Mollusca  by  the  following  characters ;  the 
lens  does  not  occupy  the  whole  of  the  cavity  of  the 
eye,  but  is  placed  anteriorly,  while  the  rest  is  filled  by 
a  vitreous  humour ;  the  lens,  therefore,  is  more 
distinctly  convex,  and  has  a  greater  influence  on  the 
impinging  rays  of  light ;  the  layer  of  rods  which 
lines  the  cavity  is  bounded  by  a  distinct  and  well- 
marked  layer  of  pigment. 

Though  the  physiology  of  the  eye  of  a  crayfish 
offers  some  considerable  difficulties  which  cannot  as 
yet  be  satisfactorily  explained,  the  morphological  series 
is  so  complete  that,  from  its  point  of  view,  much  may 
be  made  clear. 

The  prime  difficulty  lies  apparently  in  the  large 
number  of  lenses  that  seem  to  be  present  in  a  com. 
pound  eye  physiologically,  this  arrangement  is 
preceded  by  what  obtains  in  the  Chsetognath  Sagitta. 
In  this  worm,  the  eye,  which  is  completely  covered  by 
the  epidermis,  consists  of  three  biconvex  lenses,  each 


448  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

of  which  is  embedded  in  a  central  pigment  body,  and 
surrounded  by  fine  cylindrical  optic  cells,  which  form 


Figs.  191  A,  191  c.— Figures  of  Eyes  of  Arthropoda. 

A,  Eye  of  larva  of  Dytiscus,  showing  the  simplest  condition  of  a  single  layer  of 
cells  (p,  a,  r),  continuous  with  those  of  the  hypodermis  (to) ;  each  cell  is  dis- 
tinct, and  some  (r)  are  continuous  with  optic  nerve  fibres  (o) ;  I,  lens.  (After 
Grenacher.) 

c,  Simple  eye  (sternum)  of  fly,  showing  the  layer  (gk)  of  vitreous  cells  distinct 
from  the  retinal  layer  (rt) ;  c,  cuticle;  hyp,  hypodermis  ;  I,  lens  ;  fg,  fat  cells; 
tt\  trachea ;  on,  optic  nerve  ;  og,  optic  ganglion ;  st,  rods  of  retina.  (After 
Grenacher.) 

a  retina ;  each  of  these  cells  is  sharply  divided  into 
two  portions  ;  that  which  lies  nearest  the  lens  is  rod- 
like,  the  rest  is  granular  in  character.  As  the  lenses 


Chap.  XII.] 


OPTIC  ORGANS. 


449 


lie  in  different  planes  it  follows  that  light  passes  to 
the  rods  from  very  various  points. 

Among  the  Arthropoda  the  simplest  cases  are 
seen,  in  the  larvse   of  various    insects  (Fig.   191,  A); 

,f 


Figs.  191  B,  191  D.  -Figures  of  Eyes  of  Arthropoda. 


B,  A  single  cuticular  lens  of  Limulns,  to  show  the  aggregation  of  cells  to  form  a 

retinula  (rt).    (After  Lankester  and  Bourne.)    I,  lens;  rZ,  retinula;  op,  optic 

nerve. 
D,  Part  of  the  compound  eye  of  Pliri/r/anea  ;  the  retinal  cells  are  seen  to  bo  united 

into  a  retinula  (r),  which  is  differentiated  into  a  rhabdom  (m)  posteriorly  ; 

cc,  crystalline  cone  ;  /.facet  of  compound  eye  ;  pg,  pigment,  (After  Grenacher.) 

there  is  a  single  lens,  the  hypodermic  cells  that  form 
the  sensitive  elements,  and  some  of  which  are  con- 
tinuous with  filaments  of  the  optio  nerve,  are  simple 
and  separate  ;  these  cells  may  be  called  the  retinal 
cells.  This  condition  may,  as  in  Limulus  (Fig.  191  B), 
be  complicated  by  the  cells,  instead  of  remaining 
separate  from  one  another,  becoming  aggregated  into 
D  D  —  16 


450  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

definite  groups,  each  of  which  may  be  known  as  a 
retimila.  The  next  stage  in  complication  is  brought 
about  by  the  cells  of  the  sensitive  layer  becoming 
divisible  into  an  outer  (vitreous)  portion,  and  an 
inner  retinal  part ;  this  may  be  effected  without  the 
elements  passing  through  the  second  stage,  or  that 
of  segregation  into  retinulse  ;  as  happens,  for  example, 
with  the  lateral  eyes  of  the  scorpion,  and  the  more 
simple  eyes  of  adult  insects  (Fig.  191  c).  In  the 
more  complex  cases  the  retinal  cells  form  retinulse 
(Fig.  191  D). 

While  the  sensory  parts  thus  become  more  compli- 
cated, the  refractive  element,  or  cornea,  which  is  formed 
by  the  epidermis,  may  likewise  lose  its  primitive  sim- 
plicity as  a  continuous  investment  to  the  eye,  and  be- 
come divided  into  a  number  of  facets,  each  of  which 
is  in  connection  with  its  proper  set  of  sensitive  cells  ; 
and  of  these  lenses  there  may  be  several  hundreds. 
We  may  distinguish,  therefore,  an  eye  with  one  lens 
from  an  eye  with  many  by  calling  them  respectively 
monomeniscous  and  polymeniscous. 

The  eyes  of  Arthropods  (Fig.  191)  are,  therefore, 
in  the  classification  of  Lankester  and  Bourne  : 

A.  MonosticJsOMS    (formed    by    a    single    layer    of 

cells). 

o.  Non-retimilate,  as  in  the  larvae  of  insects. 
)8.  Refill  II  Sate,     i.   Lateral  eyes  of  scorpions, 
ii.  Lateral  eyes  of  Limulus. 

B.  IMjplosticllOUS  (formed  by  a  double  layer  of  cells, 

one  vitreous,  and  one  retinal). 

a.  Non-retinillate.— Dorsal  eyes  of  spiders,  and 
simple  eyes  of  adult  insects. 

£.  Retinillate. — Central  eyes  of  scorpions,  com- 
pound eyes  of  insects  and  Crustacea. 
i.  Monomeniscoiis  (with  a  single  lens). 
ii.  Polymeniscous  (with  a  number  of  lenses). 

1.  Separate  vitreoiis  bodies. 

2.  Aggregated  vitreous  bodies. 

Among  the  Invertebrata  the  highest  type  of  eye 


chap,  xii.]  EYES  OF  CHORD  ATA.  451 

is  to  be  found  in  the  dibranchiate  Cephalopoda, 

and  it  is  remarkable  for  being  protected  by  a  cartila- 
ginous orbit ;  the  sides  of  the  eye  are  protected  by  a 
hard  layer  which  has  been  called  the  sclerotic; 
this,  which,  in  front,  passes  into  the  transparent  cor- 
nea, is  either  entire,  or  perforated  in  its  centre  by 
a  more  or  less  large  aperture.  Beneath  this  is  a 
chamber  which  is  not  so  small  as  in  the  vertebrate 
eye,  and  which  sends  down  a  narrow  process  on  either 
side.  The  hinder  part  of  this  chamber  is,  in  the  long 
axis  of  the  eye,  occupied  by  the  lens,  which  is  bounded 
on  either  side  by  the  iris;  the  hinder  part  of  the 
lens  projects  into  the  hinder  or  inner  optic  chamber, 
the  posterior  wall  of  which  is  formed  by  the  retina. 
In  this  retina,  as  in  those  of  nearly  all  Invertebrates, 
the  sensitive  portion  or  layer  of  rods  is  turned 
towards  the  impinging  rays  of  light,  and  the  con- 
nective elements  are  posterior  to  it.  We  shall  shortly 
see  that  the  reverse  of  this  arrangement  obtains  among 
Vertebrates. 

The  eyes  of  the  Vertebrata  are  constantly  paired, 
and  lie,  as  an  ordinary  rule,  on  either  side  of  the  more 
anterior  portion  of  the  head  ;  they  are  always  divisible 
into  two  portions,  an  anterior  and  a  posterior  cham- 
ber, and  the  hind  wall  of  the  latter,  far  away  as  it 
lies  from  the  surface  of  the  body,  is  the  percipient 
portion  of  the  optic  organ,  and  has  an  epiblastic 
origin.  In  comparison  with  any  other  fact  as  to  the 
vertebrate  eye,  this  one  fact  stands  out  pre-eminently, 
and  first  deserves  our  attention. 

We  have  already  learnt  that  in  the  Chordata  the 
central  nervous  system  arises  as  an  infolding  of  the 
epiblasfc,  which  gradually  becomes  separated  from  the 
surface  of  the  body  ;  as  we  know,  the  result  of  this 
infolding  is  to  reverse  the  relations  of  the  outer  and 
inner  strata  of  the  epiblast,  or,  so  to  speak,  to  turn 
them  inside  out  (Fig.  192;  A,  B).  The  nervous  tube 


452  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

thus  formed  grows  out  at  its  anterior  end  into  three 
brain  vesicles  ;  from  the  foremost  of  these  an  out- 
growth is  given  off  on  either  side  which  forms  the 


D 


Fig.  192.— Diagrams  to  show  (A,  B)  the  relative  position  of  the  Eods 
and  Cones  (r)  and  the  Nerve  Fibres  (n)  in  an  Invertebrate  (A) 
and  a  chordate  Eye  (B).  c  shows  the  inpushing  of  the  Cells  fr^m 
which  r  and  n  have  their  origin,  cl  the  final  result  in  a  chonlate 
Spinal  Chord  ;  D,  the  outgrowing  Optic  Vesicle  from  the  Brain ;  E, 
the  formation  of  the  Optic  Cup. 

optic  vesicle  (Fig.  191  D)  ;  the  outer  wall  of  this 
vesicle  next  gets  pushed  in  (E),  but  the  layers  retain 
the  relations  to  one  another  that  they  had  in  the  optic 


Chap. xii.]  EYES  OF  CHORDATA.  453 

cup.  The  difference,  then,  in  the  position  of  the  rods  and 
nervous  elements  of  a  chordate  as  compared  with  that 
of  an  invertebrate  eye  is  due  to  the  primary  alteration 
in  position,  caused  by  the  mode  of  formation  of  the 
central  nervous  tube,  and  by  the  fact  that  the  retina 
is  an  outgrowth  from  an  anterior  enlargement  of  this 
tube.  The  stalk  of  the  vesicle  forms  the  optic  nerve. 

The  other  or  anterior  half  of  the  eye  has  a  his- 
tory which  is  essentially  similar  to  that  of  the  eyes 
of  most  invertebrates  ;  the  epiblast  of  the  surface 
thickens  and  gives  rise  to  the  lens  and  cornea,  while 
the  mesoblast  forms  supporting  and  protective  tissues. 

The  simple  eyes  of  the  Tunicate  agree  in  the 
important  point  of  the  position  of  the  sensory  layers, 
with  the  typical  eye  of  the  Vertebrata  ;  Ampluoxus 
has  no  well-developed  eyes,  but  eye  spots  have  been 
observed  in  the  larva ;  the  eyes  of  the  Cyclostomes 
remain  in  a  condition  which  is  embryonic  as  com- 
pared with  that  of  higher  forms  ;  a  lens,  for  example, 
is  absent.  Throughout  the  rest  of  the  Yertebrata  the 
eye  has  essentially  the  same  structure  as  in  man  (see 
"  Elements  of  Histology,"  chap,  xxxvi.,  and  "  Human 
Physiology,"  chap,  xv.)  ;  such  differences  as  obtain  are 
of  importance  and  interest  as  bearing  on  the  adapta- 
tion of  the  different  parts  of  the  eye  to  the  different 
media  in  which  it  is  placed,  or  to  certain  differences 
in  its  duties. 

In  Fishes,  where  the  aqueous  and  vitreous  hu- 
mours have  but  little  effect  in  bringing  to  a  focus 
the  rays  of  light  that  have  already  entered  the  water, 
the  antero-posterior  axis  of  the  eye  is  short,  and  the 
lens  is  very  large  and  convex,  while  the  cornea,  owing 
to  the  small  amount  of  vitreous  humour  that  is  present, 
is  much  flattened,  and,  inasmuch  as  it  is  without 
eyelids,  it  is  thereby  less  liable  to  friction  than  if  it 
projected  outwards.  The  pupil  is  large,  so  as  to  admit  a 
large  quantity  of  light ;  in  Anableps,  which  swims  with 


454  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

its  head  half  in  and  half  out  of  the  water,  the  cornea 
is  divided  into  two  halves  by  a  horizontal  line  of 
conjunctival  epithelium,  and  the  pupil  is  similarly 
divided  into  an  upper  and  a  lower  half.  The  eye  is 
kept  in  shape  and  position  by  the  cartilaginous  or 
fibrous  condition  of  the  sclerotic  ;  in  bony  fishes  plates 
of  bone  are  not  unfrequently  deposited  in  this  enve- 
lope or  coat  of  the  eye. 

The  eyes  of  the  Urodela  and  Csecilife  are  small, 
and  this  is  especially  the  case  in  the  latter  group, 
the  members  of  which  are  of  burrowing  habits;  in 
both,  the  skin  is  completely  continued  over  the  eye, 
and  in  the  Caecilise  it  is  often  quite  thick.  In  Proteus, 
the  skin  which  covers  the  lens  is  not  at  all  trans- 
parent, and  this  cave-dwelling  animal  is  so  far  blind 
that  it  is  apparently  only  able  to  distinguish  between 
light  and  darkness  ;  in  much  the  same  way,  probably,  as 
when  a  man  turns  his  face  to  the  light  and  closes  his 
eyes,  he  is  still  able  to  perceive  the  passage  of  an 
opaque  object,  such  as  a  hand,  between  himself  and 
the  light.  Just  as  the  cornea  of  fishes  is  flattened,  so 
that  of  the  amphibious  newt  and  of  the  frog  is  pro- 
vided with  a  muscle  by  which  the  eye-ball  can  be  re- 
tracted when  the  animal  is  in  tho  water. 

In  all  Reptiles  the  eyes  are  small ;  but,  partly  owing 
to  the  length  of  their  bodies,  we  are  especially  struck 
with  the  small  ness  of  the  eyes  of  Snakes  ;  in  them  the 
pupil  is  generally  rounded,  but  in  some  nocturnal 
species  it  has  the  form  of  a  vertical  slit.  There  are 
no  eyelids,  or,  in  other  words,  the  skin  is  continued 
over  the  eye,  and  this  part  of  the  integument  is  shed 
with  the  rest  of  the  skin.  In  most  other  Reptiles 
there  are  two  eyelids,  in  addition  to  the  ''nictitating 
membrane  "  which  is  found  in  some  sharks  and  Am- 
phibia, as  well  as  in  Birds,  and  which  is  drawn  over 
the  eye  by  special  muscles.  The  eye  of  the  crocodile 
is  small. 


Chap,  xii.]  EYES  OF  BIRDS.  455 

In  Birds,  even  more  than  in  Reptiles,  we  see  the 
influence  of  the  terrestrial  mode  of  life,  or  rather  of 
the  different  refractive  powers  of  air  and  water,  in 
the  more  convex  form  of  the  cornea.  It  is  clear  that 
the  rarer  the  atmosphere  the  greater  is  the  necessity 
for  a  convex  apparatus  to  collect  the  rays  of  light, 
and  we  find  an  arrangement  in  the  eyes  of  flying 
birds  by  which  this  convexity  may  be  attained.  By 
the  contraction  of  the  muscles  at  the  sides  of  and 
behind  the  eye,  the  fluids  in  its  two  chambers,  and 
thus  the  cornea,  are  pressed  forwards ;  in  some, 
pressure  on  the  optic  nerve  is  prevented,  thanks  to 
the  possession  of  bony  plates  in  the  sclerotic.  The 
crystalline  lens  is  flattened,  except  in  Apteryx  and  the 
owls  that  fly  by  twilight ;  the  ciliary  muscle,  which  is 
of  such  importance  in  the  accommodation  of  the  eye 
(see  Power's  "  Human  Physiology,"),  consists  in  birds, 
as  in  reptiles,  of  striated  muscular  tissue,  whereas  in 
mammals  the  muscle  is  of  unstriated  tissue;  owing 
to  the  difference  in  the  property  of  these  muscles,  the 
eye  of  a  swif i  ly-moving  bird  is  more  rapidly  brought 
into  focus  than  is  that  of  the  more  slowly-moving 
mammal.  It  is  not,  however,  unnecessary,  perhaps, 
to  point  out  that  this  possession  of  striated  tissue  in 
the  ciliary  muscle  of  the  eye  is  not  to  be  looked  upon 
as  a  direct  adaptation  to  the  habits  of  a  bird,  inas- 
much as  it  is  possessed  also  by  the  more  lethargic 
reptile;  all  we  can  say  of  it  is  that  it  is  a  very 
useful  heritage.  Projecting  into  the  hinder  chamber 
of  the  bird's  eye  is  a  folded  membrane  richly  provided 
with  blood-vessels  ;  this  pecten,  which  is  found  also 
in  the  eye  of  reptiles,  has  possibly  a  nutrient  function, 
but  nothing  is  certainly  known  as  to  the  office  which 
it  fills.  Compared  with  the  size  of  their  body,  the 
eyes  of  birds  are  large,  and  the  anterior  chamber  is 
remarkable  for  having  its  longitudinal  axis  as  long  as 
or  longer  than  that  of  the  hinder  chamber.  Bony 


456    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

pieces  are  found  in  the  sclerotic  of  birds,  as  of 
reptiles. 

The  eyes  of  Mammals  agree  in  essential  points 
with  those  of  man,  no  mammal  above  the  Prototheria 
having  a  bony  sclerotic ;  as  we  ascend  the  scale  we 
observe  that  the  eye  becomes  more  and  more  com- 
pletely protected,  owing  to  the  formation  by  the 
cranial  bones  of  a  bony  orbit,  which  is  to  be  seen 
in  the  dried  skull.  The  eyes  are  reduced  in  moles 
and  in  burrowing  rodents  ;  in  the  mole-rat  (Spalax) 
they  are  covered  by  the  skin.  In  those  that  seek 
their  prey  by  night  or  twilight,  the  cornea  is  larger 
and  more  convex,  and  the  pupil  broader  than  in 
the  rest;  in  such,  too,  the  lens  is  nearly  spherical. 
In  aquatic  forms,  just  as  in  fishes,  the  cornea  is 
much  less  convex  than  in  their  terrestrial  allies; 
compared  with  the  bulk  of  their  bodies,  the  eyes 
of  the  Cetacea  are  exceedingly  small. 

While  the  body  of  an  albino  is  perfectly  white, 
it  is  often  a  matter  of  astonishment  that  the  eyes 
are  red ;  but  a  little  reflection  will  show  that  this 
redness  is  due  to  the  blood  in  the  vessels  of  the 
eye,  and  that  the  colour  is  seen  in  the  eye,  though 
not  in  other  parts  of  the  body,,  in  consequence  of 
the  transparency  of  its  tissues.  It  is  not  so  fre- 
quently or  so  easily  recognised,  that  the  "  colour 
of  the  eye "  is  dependent  also  on  this  blood.  In 
light-grey  or  blue  eyes  no  pigment  is  deposited  in 
the  iris,  but  there  is  pigment  in  the  retina,  the  light 
reflected  from  which  is,  owing  to  interference,  of  a 
blue  colour.  When  pigment  is  more  thickly  laid 
down  in  the  retina,  and  becomes  also  deposited  in 
the  substance  of  the  iris,  we  have  dark-blue  or  brown 
eyes ;  and  it  is  because  tins  pigment  is  ordinarily 
laid  down  after  birth  that  we  have  the  somewhat 
strange  phenomenon  of  the  blue  eyes  of  the  babe 
becoming  brown  eyes  in  a  child. 


Chap,  xii.]  OPTIC  ORGANS.  457 

Movements  of  the  eyes.— It  is  clear  that  when 
the  number  of  eyes  becomes  limited,  the  power  of 
sight  of  their  possessor  must  either  be  very  small, 
or  the  eye  must  acquire  a  power  of  movement.  To 
obviate  this  inconvenience  various  means  have  been 
resorted  to.  The  eye  may,  as  in  polymeniscous 
Arthropods,  be  provided  with  a  large  number  of 
lenses,  so  that  the  whole  corneal  surface  extends 
over  more  than  half  a  sphere ;  in  addition  to  this, 
the  body  may  be  provided  with  less  useful  lateral 
eyes,  as  in  the  scorpion ;  or,  as  in  the  crayfish  or 
the  crab,  the  eye  may  be  placed  at  the  end  of  a 
movable  stalk.  Phenomena  of  a  corresponding  kind 
obtain  among  the  Mollusca;  the  Lamellibranchiata, 
which  are  without  prsestomial  eyes,  often  have  a 
number  of  small  eyes  (or  pigment  spots  only)  deve- 
loped on  the  edges  of  the  mantle,  and  these  even  are 
sometimes  placed  on  stalks.  In  some  Gastropods  the 
eyes  are  placed  at  the  ends  of  the  tentacles,  and  as 
these  tentacles  are  capable  of  protrusion  and  retrac- 
tion, the  optic  nerve  is  of  sufficient  length  to  be  quite 
straight  only  when  the  tentacle  is  protruded,  while, 
when  that  organ  is  retracted,  the  nerve  is  looped. 
In  Onchidium  and  some  of  its  allies,  a  number  of 
simple  eyes,  resembling  in  essential  arrangement  those 
of  the  Vertebrata,  are  developed  on  the  surface  of  the 
back  of  these  shell-less  and  slow-moving  molluscs  ; 
Semper  has  counted  as  many  as  ninety-eight  of  these 
eyes  on  the  back  of  an  Onchidium. 

In  the  Chitonidee,  Moseley  has  recently  detected 
in  some  species  more  than  ten  thousand  minute  eyes, 
placed  on  the  exposed  surfaces  of  their  shells ;  but  it 
is  remarkable  that  these  eyes,  unlike  those  of  Onchi- 
dium, are  on  the  type  of  the  invertebrate,  and  not  of 
the  vertebrate,  eye.  Scattered  among  them  are  tactile 
organs,  from  which,  it  is  supposed,  the  eyes  have 
arisen  by  modification.  In  some  of  the  heteropodous 


458    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Molluscs  the  optic  bulb  is  moved  by  muscles,  and  this 
is  the  kind  of  mechanism  which  obtains  in  the  Verte- 
brata,  where  four  rectal  and  two  oblique  muscles  are, 
as  in  man,  almost  always  developed. 

Influence  of  light  and  darkness  on  the 
development  of  the  eye.— While  our  knowledge 
of  such  terrestrial  forms  as  the  earthworm,  the 
probeus,  or  the  mole,  would  lead  us  to  think  that  a 
diminution  of  light  is  constantly  associated  with  a 
degenerate  condition  or  loss  of  the  eye,  it  is  very 
remarkable  that  among  aquatic  forms  we  find  species 
which  support,  and  others  that  unmistakably  con- 
tradict this  hypothesis.  Among  Crustacea  we  have, 
for  example,  Ethusa  granulata,  which  in  shallow 
water  has  eyes  of  the  ordinary  character,  but,  when 
taken  from  depths  of  110  to  370  fathoms,  is  found 
to  have  the  stalked  eye  replaced  by  a  calcareous 
knob  ;  it  is  a  case  to  which  the  words  of  Darwin 
are  altogether  applicable  :  "  The  stand  for  the  tele- 
scope is  there,  though  the  telescope  with  its  glasses 
has  been  lost."  Specimens  of  the  same  species, 
taken  from  a  greater  depth  (500  to  700  fathoms), 
showed  that  the  eye-stalk  had  undergone  a  change 
of  function,  and  had  become  converted  into  a 
pointed  rostrum,  which  probably  serves  as  an  organ 
of  touch.  On  the  other  hand,  species  of  Munida 
that  were  dredged  at  the  same  time  as  the  Ethusa 
were  found  to  have  exceedingly  large  eyes.  What  is 
true  of  Crustacea  is  true  also  of  fishes  ;  at  moderate 
depths  the  eyes  are  generally  large,  at  greater  depths 
they  may  be  very  large  or  very  small.  Where  the 
eyes  are  small  the  fish  has  its  tactile  organs  very 
largely  developed.  Fishes  or  Crustaceans,  however, 
taken  from  very  great  depths  (e.g.  1,900  fathoms), 
have  been  found  to  have  both  the  eyes  rudimentary 
and  the  special  organs  of  touch  absent.  There  is  no 
reason  to  suppose  that  the  so-called  eye-like  organs  of 


chap,  xii  ]  EAR  OF  MEDUSA.  459 

some  fishes  (e.g.  Scopelus)  have  any  optic  function. 
They  appear,  from  the  accounts  given  by  those  who 
have  seen  such  fishes  alive,  to  be  phosphorescent 
organs. 

The  ear.— Definite  auditory  organs  are  wanting 
in  various  lower  forms,  which  are,  so  far  as  we  can 
tell,  without  any  sense  of  hearing ;  an  intermediate 
condition,  between  that  of  absolute  incapacity  to  hear 
and  the  possession  of  this  sense,  is,  perhaps,  presented 
to  us  by  the  earless  earthworm,  on  which  the  vibra- 
tions of  air  which  are  heard  by  man  are  absolutely 
without  effect,  though  the  worms  are  very  sensitive  to 
vibrations  conveyed  along  solid  objects  ;  in  the  ex- 
periments made  by  Darwin,  "the  vibrations,  before 
reaching  their  bodies,  had  to  pass  from  the  sounding 
board  of  the  piano,  through  the  saucer,  the  bottom  of 
the  pot  and  the  damp,  not  very  compact  earth  on 
which  they  lay  with  their  tails  in  their  burrows ; " 
though  the  connection  was  so  slight,  yet  the  result  of 
striking  a  note  on  the  piano  was  the  immediate  with- 
drawal of  the  worms  into  their  burrows. 

Definite  auditory  organs  of  a  low  degree  of  or- 
ganisation are  to  be  found  in  some  Medusae,  but  it 
is  a  remarkable  fact  that,  so  far  as  we  know  at  pre- 
sent, distinct  auditory  and  optic  organs  are  never 
developed  in  one  and  the  same  species.  A  very  simple 
condition  is  found  in  such  a  jelly-fish  as  Euchilota 
or  Tiaropsis,  where  the  lower  surface  of  the  velum 
is,  at  various  points,  indented  by  open-mouthed  pits  ; 
the  sides  and  base  of  this  pit  are  formed  by  the 
epithelial  cells  of  the  velum,  and  of  these  some  on 
the  inner  face  become  provided  with  projecting  audi- 
tory hairs,  and  so  become  special  sense  cells,  while 
others  develop  within  their  contents  the  calcareous 
concretions  which  form  the  otolith ;  the  sense  cells  are 
continuous  by  their  bases  with  the  lower  nerve  ring. 
The  epithelial  cells  on  the  outer  surface  of  the  pit 


460    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

(which  belong,  of  course,  to  the  series  of  cells  that 
lie  on  the  upper  surface  of  the  velum)  have  thick 
membranes,  and  become  filled  with  fluid.  Mitrotrocha 
is  provided  with  no  less  than  eighty  such  open  pits. 

The  next  stage  in  advance  is  seen  in  Phialidium 
(Fig.  193),  where  the  pit  becomes  closed.  In  other 
Medusae  the  auditory  vesicle  appears  to  be  a  modified 


Fig.  193.— An  Auditory  Vesicle  of  Phialidium. 

d',  Epithelium  of  the  upper  surface  of  the  velum  ;  eft,  of  the  under  surface ; 
nr',  upper  nerve  ring;  ft,  auditory  cells;  hh,  auditory  hairs  ;  np,  nervous 
cushion  formed  by  a  prolongation  of  the  lower  nerve  ring  ;  r,  circular  canal 
at  the  edge  of  the  velum.  The;  spot  in  the  cavity  represents  an  otolith. 
(After  0.  and  R.  Hertwig.) 

tentacle.     In  some  cases  the  sense  organs  appear  to  be 
both  eye  and  ear. 

Throughout  the  whole  of  the  Metazoa  we  find 
that  the  auditory  organs  have  more  or  less  the  form 
of  vesicles,  which  in  lowlier  forms,  and  in  all  primi- 
tively, are  open  to  the  exterior ;  within  the  vesicles 
one  or  more  hard  bodies  are  developed,  which,  on  being 
agitated  by  the  vibrations  of  the  waves  of  sound,  act 
on  the  sensory  hairs  of  the  sense  cells,  which  are 
developed  within  the  auditory  cavity,  and  which  are 
connected  with  the  central  nervous  system  by  the 
fibres  of  the  auditory  nerve. 


Chap,  xii.]  AUDITORY  ORGANS.  461 

Among  the  Eclunodermata,  auditory  organs 
have  only  been  satisfactorily  observed  in  the  deep 
sea-dwelling  holothurian  Elasipoda,  where  they 
are  often  present  in  large  numbers,  Kolga  nana  hav- 
ing no  less  than  fifty-six  auditory  sacs;  as  in  such 
Annelids  as  possess  them,  the  sacs  are  set  close  to 
the  nerve  cords,  and  have  a  large  number  of  contained 
otoliths  or  concretions. 

We  have  more  definite  information  as  to  the 
Arthropoda  and  Mollusca;  in  the  former  they 
are  not  always  developed  on  the  head  ;  Mysis,  just  like 
Fabricia  (among  worms)  with  its  posterior  eyes, 
showing  us  that,  inasmuch  as  sensory  cells  are  distri- 
buted over  the  whole  of  the  body,  special  sense  organs 
may  be  developed  at  any  part,  and  pointing  the  moral 
by  having  auditory  organs  in  its  terminal  segment. 
As  a  rule  they  are,  as  in  the  crayfish,  developed  at 
the  base  of  the  antennules.  Here  the  auditory  sac  is 
permanently  open,  though  the  seta3  that  protect  it 
prevent  the  entrance  of  much  foreign  matter  ;  within 
this  sac  part  of  the  wall  is  raised  up  into  a  ridge,  and 
the  cells  that  form  it  are  provided  with  delicate  setae 
at  their  free  end,  and  with  nerve  fibres  at  their  base 
and  within.  The  sac  is  filled  with  a  gelatinous  fluid 
in  which  are  to  be  found  minute  otoliths ;  these  last, 
being  set  in  motion  by  vibrations  in  the  water  which 
strike  on  the  guarded  open  slit  of  the  ear  sac,  affect 
the  setae ;  the  setae  affect  the  cells  on  the  acoustic 
ridge,  and  so  the  contained  nerve  fibres  which  are 
in  direct  connection  with  the  brain. 

While  the  sac  of  the  Crustacea  resembles  in  some 
respects  the  embryonic  ear  of  the  Vertebrata,  that  of 
Insects  presents  other  points  of  similarity.  Placed 
on  the  median  segments  of  the  body  or  on  the  legs, 
the  ears  of  the  Orthoptera  are  remarkable  for  the 
possession  of  a  drum  or  tympanum  ;  this  is  merely  a 
modification  of  part  of  the  chitinous  integument  of  the 


462  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

insect,  and  the  bow  on  which  it  is  stretched  is  merely 
a  part  of  the  same  integument  that  has  become  con- 
siderably thickened.  Taking  for  an  example  the 
grasshopper,  we  find  a  number  of  small  muscles 
which  are  inserted  into  the  bow,  and  by  their  exten- 
sion or  contraction  increase  or  diminish  the  tension 
of  the  tympanic  membrane.  The  central  portion  of 
the  membrane  is  occupied  by  a  cavity  which  commu- 
nicates with  the  exterior  by  an  open  tube,  and  within 
the  cavity  there  is  a  ganglionic  mass  developed  at  the 
end  of  an  auditory  nerve.  Here,  then,  we  have  the 
case  of  the  sonorous  vibrations  impinging  on  a  mem- 
brane, which  is  held  tense ;  being  conveyed  to  an  air 
chamber,  which  is  in  relation  with  the  outer  air,  and 
is  therefore  capable  of  adapting  itself  to  any  force  that 
may  be  brought  to  bear  upon  it ;  and  being  carried 
thence  to  the  termination  of  a  sensory  nerve. 

In  some  insects,  though  not  in  the  Orthoptera, 
rows  of  corpuscles  have  been  observed  on  some  of  the 
nervures  of  the  wing,  and  as  these  are  supplied  witli 
nerve  filaments,  Braxton  Hicks  has  suggested  that 
they  are  auditory  organs  ;  somewhat  similar  organs 
found  on  the  halteres  of  the  Diptera  have  had  the  same 
function  ascribed  to  them  by  Lowne. 

In  the  ears,  as  in  some  other  parts  of  the  organi- 
sation of  the  Mollusca,  we  see  arrangements  which 
are  simpler,  and  others  that  are  more  complex  than 
those  that  obtain  in  the  Arthropoda.  The  auditory 
organ  is  often  a  simple  closed  vesicle,  surrounded  by  an 
investing  membrane,  and  having  in  its  cavity  sensory 
cells  provided  with  projecting  hairs  (Fig.  194);  the 
central  cavity  is  occupied  by  a  single  large  concretion, 
otolitli,  or  a  number  of  smaller  otoconia,  as  the 
smaller  concretions  may  conveniently  be  called.  In 
the  Nautilus,  the  ears,  as  in  most  Lamellibranchs  and 
Gastropods,  are  attached  to  the  pedal  ganglia,  but  in 
the  Dibranchiata  they  are  enclosed  in  the  cartilage 


Chap. xii.]  EAR  OF  CHORDATA.  463 

which  protects  the  cerebrum  and,  as  such  portion  of 
the  cartilage  forms  a  special  investment  for  the  ear, 
we  have  in  these  alone,  among  invertebrates,  that  dis- 
tinction between  the  outer  or  cartilaginous,  and  the 
inner  or  membranous  ear  capsule  to  which  we  are 
accustomed  in  the  ears  of  vertebrates  ;  in  Cephalopods, 
as  in  Crustacea  and  Vertebrates,  an  acoustic  ridge  or 
crest  is  formed  on  which  are  set  the  auditory  cells,  and 
in  some  of  them,  as  in  some  of  the  lower  Vertebrates,  it 
appears  that  the  ear  sac  is  permanently  in  communica- 
tion with  the  outer  world  by  a  nar- 
row open  duct,  the  remnant  of  the 
primitive  involution  of  the  epiblast 
from  which  the  organ  was  fashioned. 

Amphioxus     has     no     known 
auditory  organ,   and  that    which   is 
found  in  the  Urochordata  would 
appear  to  .have  been  independently     Fig.  194,-Diapram 
developed  within  the  limits  of   the         cycSs!   E(After 
group     (Balfour).       It    lies    on    the         Simroth.) 
under  surface   of  the  anterior  brain 
vesicle  (Fig.  195  ;  a)  ;  the  cells  of  the  brain  form  an 
acoustic  ridge,  the  delicate  hairs  on  whose  cells  hold 
an  otolith  which  projects  into  the  cavity  of  the  brain, 
and  is  remarkable  for  being  pigmented. 

Though  the  sensory  portion  of  the  ear  of  higher 
Vertebrates  is  at  some  distance  from  the  surface  of 
the  body,  it  is  not  to  be  supposed  that  it  has  not,  like 
all  other  organs  of  sense,  its  primary  seat  of  origin  in 
the  outer  layer  of  the  embryo  or  epiblast.  In  the 
lower  vertebrates  the  auditory  capsule  is  closed,  and 
lies  just  below  the  skin,  or  sinks  some  way  into  the 
walls  of  the  brain  case,  as  in  Elasmobranchs,  where 
the  duct  either  opens  to  the  exterior  by  a  minute 
pore  (ray),  or  is  closed  over  by  the  skin  (sharks)  ;  in 
the  higher  forms  a  special  auditory  passage  is  deve- 
loped. 


464  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

The  first  sign  of  the  development  of  the  ear  is  the 
formation,  on  either  side  of  the  hinder  part  of  the  head, 
of  a  pit  or  depression  (Fig.  196  ;  au  v),  which  gradually 
deepens,  and  with  which  an  outgrowth  from  the  audi- 
tory nerve  (au  n)  comes  into  contact.  The  pit  becomes 
converted  into  an  elongated  sac,  narrower  above  and 
below.  The  upper  end  grows  out  and  gives  rise  to 
the  recess  of  the  membranous  labyrinth,  the  lower 


Fig.  195. — Larva  of  Ascidia  mentula ;  the  Anterior  portion  of  the  tail  is 
alone  represented. 

Nl,  Anterior  swelling  of  neural  tube;  N,  anterior  swelling  of  spinal  portion  of 
neural  tube;  n,  hinder  part  of  neural  tube ;' c/i,  notoohorcl:  K,  {branchial 
region  of  alimentary  tract ;  d,  its  cesophageal  and  gastric  region;  o,  eye;  a, 
otolith. ;  o',  moutli ;  s,  papilla  of  attachment.  (After  Kuppffer.) 


end  forms  the  cochlear  canal,  and  the  narrow  duct 
(canalis  reuniens)  by  means  of  which  the  cochlear 
canal  communicates  with  the  sacculus  ;  the  median 
portion  of  the  auditory  vesicles  gives  rise  to  the  semi- 
circular canals,  and  to  the  utriculus. 

Fig.  197  (page  466)  will  make  clear  the  relations 
of  the  just  named  and  of  other  parts  in  the  complex 
auditory  organ  of  man. 

External  to  the  proper  sensory  portion  in  which 
the  branches  of  the  auditory  nerves  terminate  in  the 
special  sensory  cells,  there  is  a  median  division  of  the 
ear,  in  which  are  developed  one  or  more  bones  that 
convey  the  sonorous  vibrations,  and  these  are  acted 


Chap.  XII.] 


EAR  OF  VERTEBRATA. 


465 


on  by  a  membrane  (the    tympanic  membrane) 

which  is  analogous  to,  though,  of  course,  not  homo- 
logous with,  the  membrane  which  we  have  already  found 
in  the  ears  of  orthopterous  insects.  Externally  to  this 
middle  ear  is  the  outer  portion,  which  forms  the 
external  auditory  meatus,  and  is  in  higher  forms 
aided  and  protected  by  a  conch,  or  so-called  ex- 
ternal ear. 

Outer  division  of  the  ear, — The  functions    of 


awn 


-ftlEV 


Fig.  196.— Section  through  the  Head  of  a  Lepidosteus,  Six  Days  after 

Impregnation 

au  v,  Auditory  pit ;  au  n,  auditory  nerve  ;  cJi,  notochord  ;  by,  hypoblast.    (After 
Balfour.) 

the  outer  and  middle  portions  is  that  of  conveying  the 
sonorous  vibrations  to  the  sensory  region  internal  to 
them  ;  they  are  but  poorly  developed  in  the  lower 
forms  for  two  reasons  :  first  of  all,  the  insinking  of  the 
sensory  portion  which  is  so  marked  in  the  higher  forms 
is,  as  we  may  expect,  less  marked  in  the  lower  divi- 
sions of  the  Yertebrata,  for  it  is  a  process  which  has 
only  gradually  been  effected  ;  the  second  reason  is  to  be 
found  in  the  fact  that  the  trumpet-shaped  arrange- 
ments of  the  outer  region,  which  are  so  useful  in 
bringing  to  a  focus  the  vibrations  of  air,  are  not  so 
necessary  when  the  animal  lives  in  water.  Evidence 
as  to  the  influence  of  this  denser  medium  is  afforded  us 
E  E— 16 


466  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

by  the  Cetacea,  which,  at  most,  have  nothing  more 
than  an  exceedingly  rudimentary  external  ear  (pinna), 
and  this,  when  found  in  these  mammals,  is  proportion- 
ately longer  in  the  foetus  than  in  the  adult  (Howse). 


Fig.  197.— Representation  of  the  Human  Ear. 

e.  External  ear;  me,  auditory  passage  (meat us  auditorius  extermifO;  I,  bony 
labyrinth  ;  vm,  auditory  nerve  ;  o,  ossicles  of  the  ear ;  fo,  fenestra  ovalis  ; 
eu,  Eustachiah  tube,  which  opens  into  the  pharynx  ;  t,  tympanum  ;  c,  bony 
cochlea  ;  tin,  tympauic  membrane. 


Fishes  are  without  any  external  ear  or  tympanic 
membrane,  and,  as  has  already  been  said,  the  Elasmo- 
branchs  have  a  canal  which  reaches  to,  and  may  open 
on,  the  outer  surface  of  the  skull ;  in  many  Teleostei, 
where  there  is  no  such  external  opening,  there  are 
spaces  in  the  skull  which  are  only  covered  by  skin  or 
very  thin  bone  in  the  neighbourhood  of  the  ear; 
in  some  of  the  bony  fishes  there  is  an  exceedingly 


Chap,  xi LI  EAR  OF  VERTEBRATA.  467 

remarkable  arrangement,  by  means  of  which  sounds  are 
conveyed  to  the  organ  of  hearing.  The  anterior  ends 
of  the  air  bladder  are  attached  to  membranes  which 
close  in  spaces  in  the  occipital  region  of  the  skull, 
and  on  the  other  side  of  these  membranes  is  the  ear  of 
that  side.  This  simple  condition  which  obtains  in  the 
perch  and  its  allies,  is  complicated  in  carps  and  others 
by  the  addition  of  three  ossicles,  which  connect  the  air 
bladder  with  the  auditory  region,  and  convey  such 
vibrations  as  affect  the  air  in  the  air  bladder. 

In  the  lower  Amphibia  the  ear  cleft  is  merely 
closed  by  muscles,  but  in  the  Anura  there  is  a  distinct 
tympanic  membrane,  as  there  is  also  in  most,  though 
not  in  all,  Reptiles,  In  the  simplest  condition  this 
lies  on  the  surface  of  the  head,  so  that  there  is  no  ex- 
ternal auditory  meatus ;  but  in  the  lizards  we  have  a 
small  pit  external  to  the  membrane,  and  we  have, 
therefore,  the  commencement  of  an  external  ear 
passage. 

In  Mammals  the  tympanic  bone  of  the  skull 
takes  part  in  forming  the  walls  of  this  meatus.  The 
pinna  is  represented  by  a  fold  of  skin  with  combined 
muscular  tissue  in  crocodiles,  and  by  a  movable  mem- 
branous valve  in  some  birds  (owls).  It  is  not  found  in 
the  Prototheria,  but  in  all  other  Mammals  there  is  a 
well-developed  outer  ear,  which  becomes  rudimentary 
or  aborted  only  in  marine  forms.  In  many  mammals 
the  pinna  can  be  moved  by  muscles,  and  directed, 
therefore,  to  different  points  from  which  sound  is  sup- 
posed to  be  coming.  In  the  higher  Primates  these 
muscles  are  ordinarily  rudimentary,  but  their  posses- 
sion by  some  men  is  spoken  to  by  the  power  that  such 
have  of  moving  their  ears. 

Middle  car.  —  Associated  with  the  development 
of  a  tympanic  membrane  is  that  of  a  contained  tym- 
panic cavity.  This  cavity  is  not  a  new  formation,  but 
is  due  to  the  modification  of  the,  in  the  pulmonate 


468  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Vertebrates,  now  useless  visceral  clefts.  While  on  its 
outer  side  this  cleft  gives  rise  to  the  tympanic  cavity 
(Fig.  197  ;  c£)  and  part  of  the  external  meatus,  on  its 
inner  side  it  continues  to  communicate  with  the 
pharynx,  and  so  forms  the  Eustachian  tube  (e,u), 
by  means  of  which  air  can  enter  into  the  otherwise 
closed  ear  chamber.  The  vibrations  of  the  tympanic 
membrane  have  to  be  conveyed  to  the  inner  ear,  and 
this  is  effected  by  one  or  more  bones,  the  outer  end  of 
which  is  fixed  to  the  tympanic  membrane,  while  the 
other  impinges  on  the  membrane  which  closes  the  en- 
trance to  the  internal  ear  (fenestra,  ovalis,  Fig.  197). 
In  the  Amphibia  and  Sauropsida  there  is  but  a  single 
bone  in  the  middle  ear ;  the  origin  of  this  columella 
is  not  satisfactorily  established,  but  it  is,  probably,  the 
upper  end  of  the  hyoid  arch.  (See  page  327.)  In  the 
Mammalia  this  single  bone  is  replaced  by  three, 
which  are  known  respectively  as  the  stapes,  incus, 
and  malleus  ;  the  discussion  of  the  homologies  of 
these  bones  has  been  deep  and  protracted,  but  a  con- 
sideration of  it  would  be  beyond  our  scope  here.  We 
must  be  content  to  say  that,  according  to  the  latest 
views  of  Parker,  the  stapes  is  the  uppermost  element 
of  the  hyoid,  and  that  the  incus  and  malleus  belong  to 
the  mandibular  arch.  According  to  these  views  it  is 
the  incus,  and  not,  as  is  ordinarily  taught,  the  malleus, 
that  is  the  homologue  of  the  quadrate  bone  of  the 
Sauropsida.  In  the  Prototheria  the  stapes  is  not 
hollowed  out  at  one  end  so  as  to  have  the  form  of  a 
stirrup,  nor  is  the  incus  of  the  "  normal  mammalian 
form " ;  Parker  further  finds  that  the  malleus  still 
forms  the  hinder  part  of  the  mandible. 

We  have  in  this  history  of  the  auditory  ossicles 
(which  must  not  by  any  means  be  confounded  with 
otoliths)  one  of  the  most  remarkable  examples  of  the 
way  in  which  parts  useless  for  one  are  gradually  ap- 
propriated to  another  function.  The  cleft  between 


Chap,  xii.]  INTERNAL  EAR.  469 

the  hyoid  and  mandibular  arches  (the  hyomandibular 
cleft)  becoming  useless  as  an  organ  of  respiration  has 
been  seized  upon  by  the  ear ;  in  Mammals,  parts  of  the 
mandibular  arch  that  lies  in  front,  and  of  the  hyoid 
arch  that  lies  behind  the  cleft,  are  adapted  to  the  use  of 
one  and  the  same  organ. 

Internal  ear. — This,  which  is  the  essential 
portion,  as  those  just  described  are  the  accessory 
parts,  of  the  organ  of  hearing,  consists  primarily  of 
the  so-called  membranous  labyrinth,  formed  by  the 
sacculus,  utriculus,  and  semicircular  canals ;  as  we 
ascend  the  scale  we  find  a  bony  labyrinth  fashioned 
around  this  membranous  one  ;  the  space  between  them 
contains  lymph,  and  is  known  as  the  perilymphatic 
space  or  cavity,  while  the  fluid  within  the  membranous 
labyrinth  is  the  endolymph. 

The  simplest  stage  obtains  in  the  Myxinoid  round- 
mouths,  where  there  is  only  a  single  semicircular 
canal,  at  the  base  of  which  there  is,  on  either  side,  a 
swelling  or  ampulla.  In  this,  as  in  the  underlying 
vestibule,  special  auditory  cells  are  developed,  which 
are  supplied  with  filaments  from  the  auditory  nerve. 
In  the  other  division  of  the  round-mouths,  that  is,  in 
the  lampreys,  there  are  two  semicircular  canals,  and 
the  vestibule  into  which  they  open  has  two  blind 
diverticula  arising  from  it,  in  each  of  which  auditory 
cells  are  developed.  In  all  the  remaining  Vertebrata 
there  are  three  semicircular  canals. 

In  the  Elasmobranch  fishes,  and  in  the  Dipnoi,  no 
bony  labyrinth  is  formed  around  the  membranous,  but 
a  promise,  as  it  were,  is  offered  by  the  excavation  of 
the  cartilage  around  the  labyrinth,  in  a  form  not  un- 
like the  membranous  internal  ear ;  in  the  bony  fishes 
the  labyrinth  is  protected  by  bone,  but  there  is  no 
proper  bony  labyrinth.  A  further  point  of  advance  is 
to  be  observed  in  the  Ganoidei  and  Teleostei,  for  in 
them,  as  in  Chimsera,  the  gradual  insinking  of  the  ear 


470  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

is  expressed  by  the  fact  that  the  most  internal  portion 
now  projects  into  the  cranial  cavity. 

In  no  fish  is  there  anything  more  than  a  rudiment 
of  the  cochlea  (Fig.  198 ;  A,  c).  The  highest  condition 
of  the  amphibian  ear  is  seen  in  the  Anura  ;  though 
we  cannot  yet  speak  definitely  of  a  bony  labyrinth, 


Fig.  198.— Diagrams  to  show  the  Relations  of  the  Auditory  Labyrinth  in 
the  Vertebrate  Series. 

A,  Pish ;  B,  bird  ;  c,  mammal ;  u,  utriculus,  with  the  three  semicircular  canals  ! 
s,  sacculus:  c,  cochlea;  r,  aquaeductus  vestihuli ;  6,  lagena;  cr,  canalis 
reuniens.  In  c,  r  is  seen  to  divide  into  separate  passages  for  the  utriculus 
and  sacculus  ;  the  vestibule  is  seen  to  have  a  caecal  sac  at  v ;  k,  coil  of  the 
cochlea.  (Af te  r  Waldeyer.) 


yet  we  can,  in  the  frog,  see  that  the  membranous  laby- 
rinth within  has  an  influence  on  the  contour  of  the 
surface  of  its  bony  and  cartilaginous  capsule.  Two 
foramina,  the  f.  rotund um  and  the  f.  ovale,  are  now 
to  be  distinguished.  Two  tubular  outgrowths  are 
given  off  from  the  periosteum  of  the  perilymphatic 
space  ;  both  of  them  end  in  blind  sacs,  and  one  of 
them  (the  ductus  perilymphaticus)  extends  into  the 
jugular  canal,  and  part  of  the  neck  of  the  sac  is 


chap,  xii.j  EAR  OF  MAMMALS.  471 

continued  into  a  tube  which  enters  into  connection 
with  the  subarachnoid  cerebral  cavity.  Eight  different 
sensory  regions  are  now  to  be  distinguished ;  the 
cochlear  region  has  a  commencing  outgrowth  or  lagena, 
and  within  is  a  space  which  is  covered  over  by  a  very 
thin  membrane,  the  membrana  basilaris. 

In  the  Reptilia  we  observe  several  stages  in 
the  outgrowth  of  the  cochlea,  and  these  are  most 
marked  in  crocodiles,  which,  in  this  character,  as  in  so 
many  others,  stand  nearest  to  the  birds.  In  these  last 
the  lagena  is  quite  prominent  (Fig.  198;  B,  b),  and 
begins  to  take  on  a  spiral  course. 

With  the  exception  of  the  Prototheria,  all  Mammals 
have  their  cochlea  coiled  into  a  heliciform  spiral,  the 
canal  of  which  is  wound  round  the  bony  axis  or 
modiolus ;  the  coil  may  be  flat,  as  in  the  Cetacea,  or 
very  steep,  as  in  some  Rodents  (guinea- pig)  (Fig.  198  ; 
c).  The  internal  structure  of  the  cochlea  has  been 
fully  described  in  the  "  Elements  of  Histology  "  (chap, 
xii.)  ;  here  it  need  only  be  said  that  the  seal  a  vestibuli, 
the  membrane  of  Reissner,  and  the  rods  of  Corti  are 
peculiar  to  the  mammalian  ear;  as  to 
the  last,  we  have  so  far  evidence  that 
it  has  been  developed  within  the 
limits  of  the  mammalian  series  that 
we  find  them  to  be  much  more  simply 
arranged  in  the  Prototheria  than  in 
the  higher  mammals.  The  absence  of 
this  organ  from  the  ears  of  birds, 
many  of  which  are,  as  we  know, 
capable  of  being  attracted  by  musical 
sounds,  makes  it  impossible  for  us  at  present  to  accept 
the  doctrine  that  these  rods  are  physiologically  impor- 
tant as  the  means  of  distinguishing  different  notes  of 
music. 

The  otoliths  found  in  the  lymph  of  the  membranous 
labyrinth  are  ordinarily  larger  in  fishes  than  in  higher 


472  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

vertebrates;  and  their  number  is,  of  course, proportional 
to  their  size.  Most  bony  fishes  have  two  only,  but 
these  are  rather  to  be  looked  upon  as  calcareous 
masses  than  as  separate  otoliths  ;  in  Elasmobranchs 
such  otoliths  are  often  grouped  into  masses  of  various 
sizes  and  forms.  In  the  Teleostei  they  are  crystal- 
line, but  in  Chimsera  and  the  sturgeons  they  are 
more  chalky  in  character.  Their  function  is,  as  in 
invertebrates,  that  of  aiding  the  vibrating  fluid  in 
its  action  on  the  sensory  cells  of  the  auditory  crest. 


CHAPTER  XIII. 

ORGANS    OF   REPRODUCTION. 

IN  the  preceding  chapters  we  have  considered  the 
various  organs  of  the  body  by  means  of  which  an 
animal  is  enabled  to  sustain  or  defend  its  own  exis- 
tence, to  obtain  information  of  what  is  happening  around 
it,  and  to  adapt  itself  more  or  less  successfully  to  the 
course  of  events.  So  far  as  an  individual  animal  is 
concerned,  no  other  organs  than  those  with  which  we 
have  already  dealt  are  necessary  for  the  maintenance 
of  its  own  existence ;  indeed,  there  are,  we  know, 
individuals  which  do  pass  through  the  whole  of  their 
lives,  are  born,  grow,  and  die,  without  once  putting 
into  active  function  the  set  of  organs  that  remain  to 
be  considered. 

Unimportant  as  they  may  be  to  the  individual,  they 
are  of  prime  importance  for  the  species  to  which 
that  individual  belongs ;  for  they  are  the  means  by 
which  individuals  are  enabled  to  reproduce  their 
kind ;  and  they  are  of  the  more  importance  inasmuch 
as,  so  far  as  we  know,  living  matter  never  arises  or  is 
formed  except  from  pre-existing  living  matter.  In 


Chap,  xni.]      REPRODUCTION  OF  PROTOZOA.          473 

the  performance  of  that  part  of  his  life-work  which 
affects  his  race,  the  individual  reproduces  his  kind. 

This  process  of  reproduction  may  be  one  of  two 
modes,  it  may  either  be  sexual  or  asexual ;  that  is 
to  say,  two  different  cell  elements  may  unite  to  form 
a  single  cell  from  which  others  arise,  or  one  kind  of 
cell  element  alone  may  form  a  new  individual. 

The  latter  is  obviously  a  more  simple  method  than 
the  former,  and  it  is  the  only  one  which  is  certainly 
known  to  obtain  in  the  Protozoa.  Here,  too,  as 
our  previous  studies  would  lead  us  to  expect,  there  is 
no  distinct  differentiation  of  any  special  organ  ;*  we 
have  the  phenomenon,  but  not  the  organ. 

As  has  been  already  pointed  out  in  the  case  of  the 
Amoeba  (see  page  22),  the  simplest  method  of  repro- 
duction is  that  in  which  the  mass  of  protoplasm  under 
observation  divides  into  halves  of  about  the  same  size. 
Each  of  these  halves  is,  save  in  size,  a  copy  of  the 
parent;  which,  ipso  facto,  has  disappeared.  This 
method  of  reproduction  is  known  as  Fission,  and  it 
is  exceedingly  common  among  the  lower  organisms. 
In  some  cases  a  process  of  non-nucleated  protoplasm 
separates  from  the  body  of  the  Amoeba  ;  and  this  bud- 
like  outgrowth,  increasing  in  size  and  acquiring  a 
nucleus,  shortly  comes  to  have  the  form  and  structure 
of  its  parent.  This  is  the  process  by  Oemmation. 

Yet  a  third  mode  of  reproduction,  which  may  be 
called,  that  of  endo-spore  formation,  has  been  ob- 
served in  some  of  the  Protozoa ;  but  it,  like  the 
methods  of  fission  and  gemmation,  does  not  require 
the  influence  of  another  individual ;  like  them,  it  is 
absolutely  asexual.  The  protozoon,  becoming  qui- 
escent, forms  around  itself  an  envelope  or  cyst,  which 
is  at  first  transparent,  and  which  completely  encloses 

*  The  action  and  influence  of  the  nucleus  of  a  cell  is  so  obscure 
that  the  part  which  it  possibly  takes  in  initiating  cell-division 
cannot  be  discussed  in  an  elementary  work. 


474  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  rounded  cell.  The  nucleus  at  the  same  time  becomes 
invested  in  a  proper  capsule.  After  a  period  of  repose, 
the  nucleus  begins  to  break  up  into  a  number  of 
smaller  pieces,  the  wall  now  breaks,  and  each  nuclear 
portion  (spore)  uniting  itself  with  a  certain  quantity 
of  the  surrounding  protoplasm,  separates  from  the  rest 
of  the  mass,  escapes,  and  begins  to  grow  up  into  the 
form  of  the  parent  cell. 

Lastly,  we  sometimes  observe  that  two  individual 
Protozoa  of  the  same  species  become  connected  with 
one  another,  the  protoplasm  of  the  two  cells  becomes 
commingled,  the  whole  quiescent,  and  invested  in  a 
cyst.  The  enclosed  contents  break  up  into  a  number 
of  spores,  which,  on  the  eventual  bursting  of  the  cyst, 
escape  and  begin  to  grow  up  into  the  form  of  their 
parent.  Here,  then,  not  only  is  spore-formation  pre- 
ceded by  encystation,  but  also  by  conjugation.  At 
the  same  time  it  is  to  be  most  carefully  borne  in  mind 
that  the  two  individuals  are,  to  all  appearance,  essen- 
tially alike,  and  that  there  is  no  reason  whatever  for 
regarding  this  conjugation  as  being  a  sexual  act. 

The  life-history  of  the  Gregarina  presents  us  with 
a  case  of  development  by  spore  formation,  which  may 
or  may  not  be  preceded  by  conjugation.  A  single 
Gregarine,  or  two  conjugated  forms,  become  spherical, 
and  a  firm  structureless  cyst  is  gradually  developed 
around  the  protoplasm ;  the  nucleus  disappears,  and 
the  whole  of  the  contained  mass  breaks  up  into  a 
number  of  small  separate  bodies  (spores) ;  these  are 
often  spindle-shaped,  and  from  their  occasional  resem- 
blance to  the  diatom  Navicula,  have  obtained  the  dis- 
tinctive name  of  psetidonavicellse.  This  appears 
to  be  the  most  general  mode  of  spore  formation.  The 
spores  become  each  invested  in  a  distinct  envelope, 
within  which  the  protoplasm  is  contained.  The  suc- 
ceding  stages  of  development  have  as  yet  been  very 
insufficiently  studied ;  in  the  large  Gregarine  of  the 


chap,  xiii.]     DEVELOPMENT  OF  GREGARINA. 


475 


lobster   the   following     stages    have,   however,   been 
observed. 

The  protoplasm,  which  has  not  been  directly  ob- 
served escaping  from  the  spore,  is  first  seen  as  a  small 
amcebiform  and  apparently  non-nucleated  mass. 
Passing  into  a  quiescent  condition,  it  becomes  differ- 
entiated into  ectosarc  and  endosarc,  and  then  gives  rise 


Fig.  200.— Figures  of  Gregarina  of  Earthworm. 

A,  Separate  form  ;  B,  encystment  completed  ;  c,  formation  of  pseudonavicellae. 
(After  Stein  and  LieberkUun.) 


to  two  processes,  one  of  which  is  stiff,  and  the  other 
actively  motile ;  in  the  latter  granules  are  richly  de- 
veloped, and  it  is  the  first  to  become  elongated  and  to 
separate  from  the  parent  mass.  It  has  now  the  form 
and  something  of  the  movement  of  a  thread-worm, 
whence  it  has  been  called  the  pseudo  -  filaria ; 
within  this  elongated  mass  a  nucleolus  and  a  nucleus 
become  apparent,  the  tube  shortens,  becomes  divided 
into  protomerit  and  deutomerit,  and,  later  on,  deve- 
lops a  cuticle ;  so  that  we  have  here  a  minute 
example  of  the  giant  Gregarine.  The  stiff  process  has 
meanwhile  absorbed  the  remainder  of  the  parent 


47 6  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

protoplasm,  has  become  motile,  and  been  converted 
into  a  pseudofilaria. 

The  statement  that  a  kind  of  sexual  reproduction  is 
observable  among  the  Infusoria  has,  on  account  of 
the  detailed  characters  of  the  reported  observations, 
obtained  considerable  vogue.  Put  shortly,  the  account 
comes  to  this  ;  the  substance  of  the  nucleolus  becomes 
converted  into  a  number  of  curved  rods,  which  repre- 
sent the  male  element,  while  the  nucleus  breaks  up 
into  small  spherical  bodies,  which  have  been  compared 
to  ovules.  The  resulting  young  are  said  to  be  at 
first  provided  with  knobbed  tentacles  and  suckers 
(acinetiform  embryos),  and  to  become  gradually  con- 
verted into  ciliate  infusoria.  Observations  undertaken 
with  the  express  view  of  examining  into  these  results, 
have  done  anything  but  confirm  them,  for  they  have 
resulted  in  the  conviction  that  the  rod-shaped  bodies 
of  Balbiani  are  nothing  but  bacterioid  parasites 
(Vibrios),  and  that  the  so-called  embryos  are  also  para- 
sites ;  these  last,  indeed,  have,  on  direct  observation, 
been  seen  to  enter  the  body  of  one  after  escaping  from 
another  Infusorian.  What  really  does  happen  appears 
to  be  this  ;  two  individuals  (of  Paramoecium)  conjugate, 
and  remain  united  for  a  day  or  longer  ;  the  only  result 
of  this  conjugation  is  that  the  nucleus  becomes  more 
finely  granular,  while  the  nucleolus  breaks  up  into 
four  oval  capsules.  Of  these,  two  in  each  individual 
disappear,  while  the  other  two  grow  till  they  reach 
two-thirds  the  size  of  the  original  nucleus,  which  they 
then  resemble  in  appearance ;  one  of  these  remains 
as  a  nucleolus,  and  the  other  appears  to  fuse  with  the 
primitive  nucleus. 

The  process,  then,  instead  of  being  in  any  way  sex- 
ual, falls  rather  under  the  head  of  rejuvenescence ; 
the  protoplasm,  in  other  words,  seems  to  undergo  a 
kind  of  re-arrangement  in  much  the  same  way  as,  in 
the  political  world,  cabinets  sometimes  do. 


Chap.  XIII.] 


SPERMATOZOA. 


477 


Where  the  method  of  reproduction  becomes 
sexual,  we  find  sets  of  cells  or  glands  which  have  a 
different  history  and  function ;  these  are  the  male 
and  female  elements,  and  they  may  be  found  sepa- 
rately in  different  individuals  of  the  same  species,  or 
they  may  both  be  formed  in  one  individual ;  in  the 


Fig.  201.— Figure  of  Spermatozoon  of,  A,  Guinea-pig  (not  quite  mature)  ; 
B,  the  same  seen  sideways;  c,  Horse ;  D,  Newt.     (After  Klein.) 

latter  case  we  have  to  do  with  hermaphrodite  forms, 
and  these  may  be  only  structurally  hermaphrodite,  as 
are  the  earthworm  and  the  snail ;  or  they  may  also  be 
physiologically  hermaphrodite,  as  the  tapeworm,  the 
fluke,  or  the  Ascaris  nigrovenosa ;  that  is  to  say,  the 
male  elements  of  one  individual  sometimes  impregnate 
the  female  cells  of  another  individual,  and  in  other 
cases  the  two  kinds  of  sexual  cells  of  one  and  the  same 
individual  come  into  union.  It  ordinarily  happens 


478  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


-c 


that  the  set  of  cells  which  give  rise  to  one  are  quite 
separate  from  those  which  give  rise  to  the  other  sexual 
cells,  but  this  is  not  always  the  case,  as  the  herma- 
phrodite gland  of  the  snail  and  the  generative  cells  of 
the  just-mentioned  Ascaris  are  sufficient  to  bear 
witness. 

The  broad  differences  between  a  male  and  a 
female  element  may  be  easily  apprehended ;  starting 
from  cells  which  are  essentially  similar  in  character, 
those  which  are  to  become  the 
male  bodies  subdivide,  and 
each  cell  gives  rise  to  a  large 
number  of  smaller  bodies, 
which  typically,  though  by  no 
means  always,  consist  of  a 
rounded  head  (which  repre- 
sents part  of  the  nucleus  of 
the  original  cell),  and  a  more 
or  less  long,  actively  moving 
tail  (Fig.  201).  The  female 
cell  (Fig.  202),  on  the  other 
hand,  increases  rather  than 
diminishes  in  size,  and  often 

acquires  considerable  bulk  from  the  large  number  of 
yolk  cells  that  are  aggregated  around  it ;  it  frequently 
also  becomes  invested  in  a  membrane,  the  outermost 
portion  of  which  may,  as  in  the  familiar  example  of 
the  egg  of  the  bird,  form  the  basis  for  a  shell,  which 
may  be  calcareous  or  chitinous. 

The  generative  cells  are,  in  their  simplest  condition, 
nothing  more  than  modified  elements  of  the  epithelial 
layer  which  lines  the  body  cavity,  and  it  is  only  with 
increasing  differentiation  of  structure  that  they 
become  aggregated  into  definite  masses  holding  a 
certain  topographical  relation  to  the  other  parts  of 
the  organism.  The  influence  of  the  male  on  the 
female  element  will  be  described  shortly  (page  482). 


Fig.  202.— Eipe  Ovtun  of  Cat. 

a,  Zona  pellucida ;  b,  germinal 
vesicle ;  c,  protoplasm. 
(After  Klein.) 


Chap,  xiii.]  SPERMATOGENESIS.  479 

We  must  first  develop  in  detail  the  characters  of  the 
parts  whose  general  morphology  has  just  been 
sketched. 

It  is  only  lately  that  much  attention  has  been 
given  to  spermatogenesis,  or  the  history  of  the 
development  of  the  spermatozoa,  and  it  will  be  most 
convenient,  therefore,  to  give  an  account  of  a  common 
form  (the  earthworm)  in  which  the  process  seems  to 


Fig.  203. — Figures  showing  the  Mode  of  Development  of  the  Sperma- 
tozoon of  the  Earthworm.  A,  Spermatospore ;  B,  5Toung  Sperma- 
tosphere,  with  eight  Spefmatoblasts  ;  c,  Spermatoblasts  with 
Central  Blastophore ;  D,  Spermatoblasts  with  protruding  Filament 
(After  J.  E.  Blomfield.) 


have   been   worked   out   (by   Blomfield)    with    great 
exactness. 

The  testis  of  the  earthworm  is  a  body  of  irregularly 
quadrate  form,  which  is  about  one-tenth  of  an  inch  in 
diameter,  and  is  directly  attached  to,  and  seems  to 
form  a  modified  part  of,  the  epithelium  lining  the 
body  cavity;  it  consists  of  a  mass  of  cells,  each  of 
which,  breaking  away  from  the  common  mass,  makes 
its  way  into  a  special  reservoir,  there  to  undergo  its 
further  development.  Each  of  these  cells  may  be 
known  as  a  spermatospore,  and  is  distinguished 
by  the  comparatively  large  size  of  its  nucleus,  and  its 
thin  coat  of  surrounding  protoplasm ;  the  nuclei  of 
these  spermatospores  undergo  division,  and  the  whole 
mass  increases  in  size.  When  eight  segments  have 
been  thus  formed  we  get  the  spermatospliere, 


480  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

which  consists  of  eight  spermatoblasts,  with  a 
small  central  mass  of  inactive  protoplasm  (folasto- 
phore).  Division  of  the  cells  still  continues,  until  at 
last  we  get  a  spermatosphere,  which  consists  of  a 
number  of  elongated  spermatoblasts  supported  by  the 
blastophore  (Fig.  203  ;  c).  The  protoplasm  around 
the  nucleus  of  each  spermatoblast  next  collects  into  a 
small  cap,  and  then  gives  off'  a  delicate  filamentous 
process  (Fig.  203  ;  D),  which,  gradually  increasing  in 
size,  comes  to  form  the  tail  of  the  spermatozoon. 
Further  changes  in  form  are  effected,  and  the  con- 
stituent spermatoblasts  of  the  sphere  fall  away  from 
one  another,  to  become,  each  of  them,  an  actively 
motile  spermatozoon  capable  of  fertilising  a  female 
cell. 

The  essential  points  in  this  history  have  been 
detected  by  various  observers  in  other  animals,  many 
of  whom  have,  however,  somewhat  obscured  the 
subject  by  the  number  of  new  technical  terms  which 
they  have  introduced. 

Oogenesis,  or  the  development  of  ova,  has 
been  more  thoroughly  studied  than  spermatogenesis, 
but  the  subject  is  rendered  more  complicated  by  the 
fact  that  the  egg  cell  either  absorbs  in  early  periods, 
or  is  for  a  time  surrounded  by  nutrient  or  yolk  cells. 
The  egg  cells  of  the  earthworm  form  a  coherent  mass, 
which  occupies  a  similar  position  in  the  thirteenth  to 
that  occupied  by  the  testes  in  the  tenth  and  eleventh 
segments,  and  is  only  distinguished  by  the  investment 
of  firm  membrane,  which  surrounds  the  mass  of  cells, 
or  ovary,  and  separates  it  from  the  rest  of  the 
epithelium  of  the  body  cavity ;  the  constituent  cells 
of  this  ovary  do  not,  however,  undergo  the  segmenta- 
tion which  affects  the  male  elements. 

Consisting,  in  the  simplest  cases  (e.g.  Hydra),  of  a 
naked  mass  of  protoplasm,  the  ovum,  with  its  nucleus 
(here  called  germinal  vesicle)  and  nucleolus  (or 


Chap,  xiii.]       HISTORY  OF  THE  OVUM.  481 

germinal  spot),  brings  to  our  mind  the  Amoeba, 
with  which  our  studies  commenced  ;  and,  if  we  observe 
its  early  behaviour,  we  are  the  more  struck  with  the 
resemblance,  for  we  often  find  it  seizing  on  and 
making  part  of  itself  the  cells  which  surround  it.  In 
the  great  majority  of  cases  the  cell  becomes  so  far 
differentiated  that  it  develops  around  itself  an  in- 
vesting (vitelliiie)  membrane  ;  here,  again,  recalling 
to  mind  the  next  stage  in  protozoic  differentiation  in 
so  far  as  protoplasmic  pseudopodial  processes  pass 
through  the  pores  in  the  membranous  wall  (Toxo- 
pneustes).  In  more  elaborated  stages  the  surrounding 
cells  of  the  ovary  give  rise  to  more  specialised 
membranes,  and  in  some  cases  it  appears  to  be 
necessary  to  leave  an  orifice  (so-called  "  micropyle  "), 
by  means  of  which  nutrient  material  or  fertilising 
elements  may  be  allowed  to  enter  and  come  into 
contact  with  the  substance  of  the  egg. 

The  final  act  in  the  maturation  of  the  ovum 
appears  to  be  the  extrusion  of  the  two  polar 
globules.  The  nucleus  of  the  egg  cell  (the  ger- 
minal vesicle)  moves  towards  the  periphery  of 
the  cell ;  as  it  does  so  its  surrounding  membrane 
becomes  absorbed,  and  the  contents  altered  in 
character.  What  remains  of  the  germinal  vesicle 
becomes  spindle-shaped,  and  one  end  of  the  spindle  is 
protruded  from  the  edge  of  the  cell.  The  projecting 
portion  is  next  constricted  from  the  rest,  and  so  gives 
rise  to  the  first  polar  globule.  The  process  is  again 
repeated,  a  second  spindle  being  formed,  and  the 
projection  being  again  constricted  off"  to  give  rise  to 
the  second  polar  globule. 

Whatever  be  the  real  explanation  of  this  pheno- 
menon, it  is,  in  the  first  place,  clear  that  it  bears 
a  very  striking  analogy  to  what  happens  in  the 
male  cell,  where  a  portion  of  the  original  protoplasm 
becomes  the  inactive  blastophor ;  and  we  can  hardly 
FF— 16 


482  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

help  giving  in  our  adhesion  to  so  much  of  the 
doctrines  of  Sabatier  as  are  contained  in  his  thesis 
that  all  cells  are  originally  hermaphrodite,  and  that 
some  by  losing  one  element,  and  some  the  other, 
become  distinctively  male  or  female  cells.  Balfour 
has  enunciated  the  striking  and  bold  hypothesis  that 
the  function  of  forming  polar  cells  was  acquired  by 
the  ovum  with  the  object  of  preventing  partheno- 
genesis ;  the  strongest  support  for  this  doctrine  was 
found  by  Balfour  in  the  reported  absence  of  polar 
globules  in  the  only  two  divisions  of  the  animal 
kingdom  (Rotifera  and  Arthropoda)  in  which  we 
normally  find  development  of  ova  without  male 
influence.  On  the  other  hand,  Billet,  with  a  full 
knowledge  of  the  facts,  and  of  Balfour's  hypothesis, 
has  lately  recorded  the  presence  of  polar  globules  in 
the  Rotifera,  and  Grobben  has  given  a  less  pointed 
account  of  the  formation  of  the  same  bodies  in  some 
of  the  lower  Crustacea.*  The  remaining  portion  of 
the  original  nucleus  returns  to  the  centre  of  the  egg, 
where  it  forms  the  female  promicleiis.  The 
mature  egg,  or  female  element,  requires  the  addition 
of  the  male  element  or  spermatozoon,  before  it  can 
set  out  on  the  course  of  its  development.  When 
brought  into  the  neighbourhood  of  the  male  cell  we 
find  that  an  egg  will  receive  one  or  more  spermatozoa, 
but  that,  if  fresh  and  uninjured,  not  more  than  two  or 
three  pass  into  it ;  if  they  do  the  future  of  the  egg  is 
endangered.  As  a  rule,  only  one  spermatozoon  enters 
into  and  becomes  a  constituent  of  the  protoplasm  of 
the  ovum  ;  the  tail  of  the  male  cell  disappears,  but  its 
head  persists  for  a  time  as  a  distinct  structure ;  this 

*  The  student  will  not  fail  to  observe  that,  at  the  present  time, 
a  well-conducted  and  carefully  described  series  of  observations  on 
a  selected  form  may  affect  very  deeply  the  speculations  of  previous 
students.  Conversely,  philosophical  speculations  have  a  guiding 
influence  on  lines  of  study. 


Chap,  xiii.]    REPRODUCTION  OF  SPONGES.  483 

may  be  call  the  male  proiiucleus.  Approaching 
the  female  pronucleus  it  gradually  fuses  with  it,  and 
thereby  gives  rise  to  a  fresh  structure,  the  so-called 
segmentation  nucleus. 

Pausing  for  a  moment  to  consider  how  far  the 
history  now  detailed  has  led  us,  we  find  that  there  has 
been  a  fusion  of  cells  which,  although  different  in 
final  form,  have  arisen  from  parts  which  at  first  were 
exactly  similar. 

In  the  lowest  forms  the  generative  cells  are  not 
aggregated  into  any  special  masses,  and  though  we 
can  say  that  here  there  are  male  and  there  female 
cells,  we  cannot  with  accuracy  speak  either  of  testes 
or  of  ovaria ;  here,  as  with  various  other  organs,  we 
find  a  diffused  preceding  a  localised  or  concentrated 
arrangement. 

The  Sponges  afford  an  example  of  this,  the 
reproductive  cells  being,  as  a  rule,  scattered  through 
the  mesoderm  (see  Fig.  53,  page  106)  ;  to  this  state- 
ment Myxospongia  and  Euspongia  form  exceptions  ; 
in  the  latter  the  ova  are  arranged  in  small  groups,  are 
embedded  in  connective  tissue,  and  hold  a  definite 
topographical  relation  to  the  afferent  canals.  Here, 
too,  the  ova  are  naked  and  amoeboid,  and  not  yet 
enclosed  in  a  distinct  membrane,  as  they  are  in  most 
of  the  higher  Metazoa. 

Asexual  reproduction  does  obtain  so  far  among 
the  sponges  that  buds  may  be  given  off  from  an 
individual,  and  an  increase  in  a  sponge  colony  can  be 
effected  in  a  way  which  is  of  some  commercial  impor- 
tance. The  method  here  referred  to  has  been  tried  in 
the  Mediterranean,  and  in  the  Florida  sponge  fishery 
with  a  certain  measure  of  success,  the  greater  com- 
pleteness of  which  does  not  appear  to  depend  on  the 
sponge  as  much  as  on  suitable  fishery  legislation.  A 
piece  of  sponge,  some  two  or  three  inches  high,  is 
carefully  cut  off  from  the  rest  of  the  mass  ;  owing,  as 


484  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

it  seems,  to  the  injury  done  to  the  sponge  by  the 
operation,  no  growth  occurs  during  the  first  four 
months,  but  during  the  next  two  months  it  will  be 
found  to  have  grown  two  or  three  inches. 

Taking  the  groups  in  order,  we  find  a  higher  grade 
of  localisation  in  the  Ccelenterata  than  we  should 
have  been  led  to  expect  from  what  we  know  of  the 
sponges.  In  Hydra,  for  example,  the  testes  are 
always  placed  just  below  the  circle  of  tentacles,  and 
the  ovary  nearer  the  foot ;  in  the  sea-anemone  the 
generative  glands,  or,  as  they  may  be  more  shortly 
called,  the  gonads,  are  developed  on  the  sides  of  the 
primary  septa ;  in  the  jelly-fishes  they  are  found  on 
the  walls  of  the  gastro-vascular  canals.  It  is  clear, 
then,  that  there  is  localisation,  but  this  is  still  of  a 
diffused  nature  \  the  generative  elements  are  not,  as 
in  the  crayfish  or  the  fowl,  limited  to  one  aggrega- 
tion, but  there  are  several  cell  aggregates,  each  with 
a  reproductive  function. 

This  phenomenon  is  most  striking  in  the  case  of  a 
colony  of  hydroid  polyps,  such  as  that  presented  by 
Syncoryne.  Here  we  find  that,  of  the  numerous 
buds  developed  on  the  colony,  some  never  attain 
to  nutrient  functions,  and  never  have  the  oral  cone 
or  tentacles  of  a  nutrient  person  (trophosome) ; 
instead  thereof,  they  become  gradually  fashioned  into 
the  shape  of  bell-like  Medusae,  separate  from  the 
colony,  become  free-swimming,  and  develop  gonads 
on  the  walls  of  their  gastro-vascular  canals.  In  other 
cases  the  medusoid  buds  or  gonosomes  become 
more  or  less  completely  developed,  but  never  separate 
themselves  from  the  rest  of  the  colony  ;  within  such 
buds  gonads  become  developed. 

This  method  of  division  of  labour,  some  persons 
of  the  colony  undertaking  nutrient  and  others  gene- 
rative functions,  is,  as  may  be  supposed,  particularly 
well  seen  in  the  Siphonophora,  where  special  sets  of 


Chap.  XIII. ]  GONADS    CF    CtSTODA.  485 

persons,  more  or  less  medusoid  in  form,  devote  them- 
selves solely  to  the  duty  of  producing  genital  glands, 
and  obtain  the  necessary  food  from  the  nutrient 
persons  of  the  colony  ;  in  a  few  cases  these  gonosomes 
become  free. 

The  Platyhelmintnes  present  an  elaborate 
and  somewhat  difficult  arrangement  of  their  sexual 
organs  ;  this  is  no  doubt  to  be  partly  explained  as  due 
to  their  exhibiting  an  early  stage  in  the  consolidation 
of  the  diffused  reproductive  cells ;  we  must,  how- 
ever, not  fail  to  note  that  they  present  a  distinct 
advance  in  the  possession  of  accessory  repro- 
ductive organs.  The  male  is  provided  with  a 
copulatory  organ  or  penis,  and  the  female,  which 
may  now  have  special  ovarian  ducts,  has  the  ter- 
minal portion  of  the  efferent  tube  modified  into  a 
special  canal  (vagina),  into  which  the  male  organ 
may  be  received.  Nor  is  this  all ;  another  portion 
of  the  duct  is  widened  out  into  a  receptacle  in  which 
the  ova  may  pass  through  the  earlier  stages  of  their 
development  (uterus) ;  and  yet  another  is  often 
converted  into  a  pouch,  in  which  the  male  elements 
may  be  stored  till  such  time  as  the  ova  are  ready  for 
fertilisation  (receptaculum  seniinis).  The  egg- 
producing  and  the  yolk-producing  cells  are,  however, 
still  distinct,  and  the  latter  have  not  yet,  as  in  the 
case  of  a  bird,  for  example,  taken  their  place  on  some 
part  of  the  duct  that  leads  from  the  ovaries. 

This  kind  of  arrangement  is  well  seen  in  the  Cestoid 
Bothriocephalus  latus  (Fig.  204;  AB),  where  the  testes 
(t)  are  seen  to  be  represented  by  aggregations  of  cells 
which  are  scattered  through  each  segment  j  their  pro- 
ducts pass  by  narrow  ducts  (ve)  into  a  common  coiled 
efferent  vessel  (vd)  which  opens  at  the  anterior  end  of 
the  segment  into  the  copulatory  organ  (cirrus,  c).  The 
ovarian  region  (ov)  occupies  either  side  of  the  middle 
line  in  the  hinder  region  of  the  segment,  while  the 


486  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


yolk-producing  glands  (d)  lie  outside  of  these ;  from 
the  latter  there  are  given  off  a  number  of  fine  canals, 
which  meet  in  the  middle  line  (d')  towards  the  hinder 
end  of  the  segment ;  these  canals  communicate  with 


?©  t 


e 

t 

® 
tm 


•3 
• 

V' 


Fig.  204  A. — Male  Apparatus  of  Bothriocephalus  latus. 

t,  Some  of  the  testes,  with  (ye)  their  ducts  opening  into  (vd)  the  vas  deferens  ; 
c,  cirrus  ;  cbt  cirrus  sheath;  ov,  ovaries;  u,  uterus.  (After  Sommer  and 
Landois.) 

the  oviduct  (od),  which  leads  into  a  large  coiled 
uterus  (u),  which  opens  near  the  anterior  end  of  the 
joint  (u'J. 

The  liver  fluke  again  presents  us  with  a  very 
complex  arrangement  of  parts  ;  there  are  two  testes, 
one  of  which  is  placed  in  front  of  the  other  in  the 
middle  of  the  body  j  each  consists  of  a  large  number 
of  blind  tubes  of  varying  lengths,  which  open  into 


Chap.  XIII.] 


Go  NADS  OF  FLUKE. 


three  or  four  primary  ducts  ;  these  unite  at  a  common 
point,  one  for  each  testis.  Thence  there  pass  forwards 
two  deferent  ducts,  which  open  into  a  common  seminal 
reservoir.  The  reservoir  is  continuous  anteriorly  with 


Fig.  204  B. — Female  Apparatus  of  Bothriocephalus  latus. 

c.  Cirrus;  cb,  cirrus  sheath;  ov,  ovaries;  d,  yitellaria ;  d',  their  ducts;  od, 
oviduct ;  u,  uterus ;  u',  its  orifice ;  v,  vagina ;  v',  its  orifice  ;  gl,  shell- 
producing  glands.  (After  Sommer  and  Landois.) 

a  ductus  ejaculatorius,  which  is  coiled  and  looped,  and 
which  forms  a  papilliform  projection  at  the  base  of 
the  genital  sinus,  which  is  common  to  the  male  and 
female  ducts,  and  is  placed  just  behind  the  ventral 
sucker.  The  terminal  portion  is  protrusible,  and 
functions  as  a  penis ;  it  is  ordinarily  known  as  the 
cirrus.  In  the  female  organs  the  germ  glands  are 
quite  distinct  from  the  yolk  glands ;  the  former  make 


488  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

up  a  single  mass,  which  lies  well  forwards,  and  is 
composed  of  a  number  of  blind  tubes ;  the  whole 
ovary  is  not  very  large.  The  yolk  glands  occupy  the 
sides  of  the  body,  extend  farther  forwards  than  the 
ovary,  and  reach  to  quite  the  hinder  end  ;  they  are 
racemose  in  character,  each  lobule  being  made  up  of 
a  number  of  small  vesicles,  the  ducts  from  which  open 
into  a  large  longitudinal  duct,  of  which  there  is  one 
on  either  side  of  the  body.  Just  below  the  level  of 
the  ovary  each  of  these  gives  off  a  transverse  duct, 
which  opens  into  a  common  median  reservoir ;  the 
duct  from  this,  after  a  somewhat  irregular  course, 
during  which  it  gives  off  the  fine  duct  of  Laurer  and 
Stieda  which  opens  by  a  very  minute  pore  on  the 
dorsal  surface  of  the  body,  opens  into  the  genital 
sinus ;  the  terminal  part  of  the  duct  serves  as  a 
vagina.  At  the  commencement  of  its  course,  where 
it  unites  with  the  short  oviduct,  it  is  surrounded  by  a 
complex  of  glands,  the  so-called  shell  glands,  which 
are  so  called  from  their  secretion  serving  to  form  a 
shell  for  the  ova. 

In  the  round  worms  (Neniatohelniintlies)  the 
sexes  are  nearly  always  separate,  and  the  generative 
products  are  developed  in  special  tubes,  which  in  the 
female  are,  however,  of  considerable  length.  Owing 
to  their  length,  the  female  tubes  are  in  most  cases 
coiled,  but  this  is  the  only  character  which  presents 
any  complexity ;  the  blind  end  of  the  tube  serves  as 
the  seat  of  production  of  the  egg  cells,  and  the  re- 
mainder has  the  function  of  an  uterus,  or  of  a  vagina. 
Here,  again,  we  find  that  while  the  blind  ovarian 
portions  of  the  tubes  are  double,  the  efferent  portion 
(vagina)  has  undergone  fusion,  and  the  generative 
orifice  is  therefore  single  and  median.  The  male 
tubes  have  essentially  the  same  structure,  and  are 
only  less  complicated  in  character ;  the  blind  end 
of  each  tube  is  the  seat  of  development  of  the 


Chap.  XIII.]  GONADS   OF    WORMS.  489 

male  cells,  while  the  rest  forms  merely  an  efferent 
duct,  or  has  its  distal  portion  widened  out  into  a 
reservoir  or  seminal  vesicle.  The  male  orifice  is, 
however,  associated  with  that  of  the  intestine,  and 
two  chitinous  spicules  are  developed  to  serve  as  copu- 
latory  organs,  and  to  aid  in  the  entrance  of  the 
spermatozoa,  which  here  are  always  amoeboid  in  form, 
and  have  no  specialised  mobile  tail. 

The  Acanthocephali  are,  again,  examples  of 
forms  in  which  the  sexes  are  separate  and  the  males 
provided  with  a  copulatory  organ,  but  they  exhibit  so 
much  advance  in  structure  as  is  implied  by  the  testes 
being  two  definite  sacs;  these,  however,  are  not 
paired,  but  one  lies  in  front  of  the  other  ;  the  testes, 
like  the  ova,  are  developed  on  a  special  cord 
(ligamentum  suspeiisoriuiii),  the  exact  signifi- 
cance of  which  is  very  incompletely  understood  ;  a 
somewhat  similar  structure  has  been  observed  in  the 
Bryozoa ;  in  the  Chsetognatha  we  find  that  the  testes 
are  developed  in  the  anenterous  or  caudal  segment 
of  the  body,  and  the  ova  in  the  segment  in  front ;  in 
the  Rotifer*  the  males  are  always  without  an 
intestine. 

In  the  Nemertinea,  the  Oephyrea,  and  the 
polychaetous  Chsetopods  the  generative  pro- 
ducts are  developed  directly  from  the  epithelial  cells 
lining  the  body  cavity,  and  there  is  no  definite  region 
of  which  one  can  speak  as  testicular  or  ovarian ;  in 
these  cases,  moreover,  the  sexes  are,  as  a  rule,  sepa- 
rate ;  and  we  have  herein  some  support  for  the  view 
that  the  hermaphroditism  of  many  worms  has  been 
secondarily  acquired. 

In  the  Hirudinea  and  the  oligoehsetous 
Cheetopods  we  find,  on  the  other  hand,  that  the 
generative  products  are  developed  in  certain  segments 
only,  and  here,  too,  we  find  that  the  two  sexes  are 
united  in  the  same  individual.  Taking  as  types  of  these 


49°  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

two  groups  the  leech  (Hirudo)  and  the  earthworm 
(Lumbricus),  we  have  a  striking  example  of  the  way 
in  which  more  lowly  and  more  specialised  characters 
are  often  associated  in  the  same  individual.  Starting 
with  the  proposition  that  the  generative  organs  are  at 
first  irregularly  distributed  through  the  body  cavity, 
we  are  led  to  suppose  that  the  more  these  sexual  cells 
are  consolidated,  or,  in  other  words,  the  fewer  the 
segments  in  which  they  are  developed,  the  higher  the 
grade  of  organisation.  From  this  point  of  view  we 
should  assign  the  higher  position  to  the  earthworm, 
inasmuch  as  the  testes  are  to  be  found  in  two  seg- 
ments only,  while  the  leech  has  testes  developed  in 
nine  segments ;  on  the  other  hand,  the  arrangements 
for  the  safe  disposal  of  the  testicular  products  are  no 
less  indicative  of  superior  organisation;  from  this 
point  of  view  we  should  assign  the  higher  position  to 
the  leech,  inasmuch  as  it  has,  and  the  earthworm  has 
not,  a  special  intromittent  organ,  or  penis,  by  means 
of  which  the  male  products  are  safely  carried  to  the 
female. 

Notwithstanding  the  absence  of  a  penis  in  most 
Oligochsetes,  different  individuals  copulate  with  one 
another,  and  the  male  products  of  the  one  are  received 
by  the  other  into  special  pouches,  whence  they  are 
expelled  when  the  ova  are  mature  and  expelled  also, 
while  some  of  the  setse  in  the  region  of  the  body 
where  the  orifices  are  placed  are  specially  modified  to 
aid  in  copulation.  The  spermatozoa  are  often  collected 
into  masses  or  spermatophores,  which  may  or  may 
not  be  provided  with  a  special  investment. 

While  the  generative  products  of  the  Nemertinea  es- 
cape directly  to  the  exterior,  and  those  of  the  Gephyrea, 
and  probably  also  of  some  Annelids,  by  means  of  the 
nephridial  canals,  the  Hirudinea  and  some  Oligochseta 
are  provided  with  special  ducts  ;  in  the  earthworm 
the  open  funnel-shaped  orifice  of  the  efferent  duct 


Chap.  XIII.]  GONADS    OF    WORMS.  49! 

becomes  greatly  enlarged,  and  two  pairs  of  large  sacs 
(the  already  mentioned  reservoirs),  are  developed, 
which  completely  obscure,  and,  indeed,  have  been 
very  frequently  mistaken  for  the  true  testes.  The 
only  explanation  which  has  been  given  of  the  pre- 
sence of  nephridial  canals  and  efferent  ducts  in 
the  same  segment  is  that  of  Lankester,  who  has  sug- 
gested that  each  segment  was  typically  provided  with 
two  pairs  of  segmental  organs,  the  superior  of  which 
ordinarily  become  aborted.  Some  worms  (Eudrilus), 
present  indications  of  the  presence  of  both  sets  of 
organs,  and  in  others  (Ocnerodrilus),  the  ordinary 
nephridia  are  not  developed  in  the  segments  which 
carry  the  generative  ducts. 

While  both  the  leech  and  the  earthworm  have  but 
a  single  pair  of  ovaries,  the  former  is  provided  with  a 
single  median  vagina,  in  place  of  a  duct  opening 
directly  to  the  exterior ;  in  the  leech,  moreover,  the 
ova,  when  set  free  from  the  ovary,  are  not  taken  up  by 
the  open  mouths  of  the  oviducts,  and  as  the  wall  of 
the  oviduct  is  directly  continuous  with  the  investment 
of  the  ovary,  they  do  not,  as  in  most  vertebrates,  for 
example,  pass  first  into  the  body  cavity  ;  they  make 
their  way  directly  to  the  exterior. 

As  may  be  supposed  from  their  radial  symmetry 
in  adult  life,  the  Echinodermata  have  their  genera- 
tive organs  radially  disposed ;  in  the  more  primitive 
forms,  such  as  Brisinga,  the  glands  are  arranged  by 
pairs,  and  extend  along  the  whole  length  of  each  arm, 
while  the  generative  pore  is  placed  in  the  proximal 
region  of  each  ray  ;  as  centralisation  increases,  the 
glands  become  less  elongated,  and  the  pores  are  placed 
within  the  area  of  the  disc  ;  in  the  Ophiuroids  the 
glands  are  completely  confined  to  the  disc,  where  they 
form  five  racemose  groups  ;  in  the  Echinoids  the  in- 
terradially-placed  pairs  become  fused,  and  only  five 
sets  of  genital  glands  are  to  be  made  out.  In  the 


492  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Holothurians  reduction  is  carried  still  farther,  the 
genital  tubes  finally  uniting  into  a  single  tube  which 
opens  near  the  anterior  end  of  the  body. 

In  keeping  with  their 
other  primitive  characters, 
the  Crinoidea  have  a  more 
diffused  arrangement  of 
the  genital  glands  ;  these 
are  situated  in  the  axis  of 
each  arm,  and  make  their 
way  into  the  pinnulse  that 
are  attached  to  it  (Fig. 
205). 

It  is  ordinarily  sup- 
posed that  the  generative 
products  of  all  Echino- 
derms  make  their  way  in  to 
the  water  in  a  more  or  less 
casual  manner ;  in  Aste- 
rina,  however,  Ludwig  has 
observed  that  the  males 
twist  their  arms  around 
those  of  the  female,  and  so 
dispose  themselves  as  to 
ensure  the  escaping  sper- 
matozoa meeting  with  the 
ejected  ova.  A  somewhat 
similar  mode  of  copulation 


Fig.  205.— Cross  Section  of  a  Pinnule 
of  the  Arctic  Feather-star  (An- 
tedon  eschrichti]  ;  x  75. 


has  been  observed  by  Jic- 
keli  in  Antedon  rosacea. 
In   addition  to  their 


a,  Axial  cord;  a',  its  branches  ;  ag,  ara- 
bulacral   or  food  groove  ;    b,  radial 
blood-vessel;    w,  radial   nerve;  ov, 
ovary  ;  pj,  joint  of  pinnule  ;  w,  water- 
vessel  ;  T,  tentacles.  (From  P.  H.  Car-  .,  ..  . 
penter.slightly  altered  from  Ludwig.)      power    Ol      SCXUal     multi- 
plication, the   Echinoder- 

mata  are  distinguished  by  their  remarkable  capacity 
for  repairing  injuries,  and  of  giving  rise  to  new  indi- 
viduals from  separated  arms ;  in  those  cases  in  which 
the  rays  are  very  numerous  (Brisinga,  Labidiaster), 


chap,  xiii.]    REPRODUCTION  OF  ECHINODERMS.    493 

it  would  appear  that  the  arms  often  break  off  for  the 
purpose  of  more  effectually  evacuating  their  genital 
products.  In  some  cases,  ordinary  five-rayed  forms, 
such  as  the  common  starfish,  have  been  seen  to, 
as  it  were  spontaneously,  break  off  an  arm ;  from 
such  a  single  arm  several  new  rays  are  budded  off,  and 
as  these  only  gradually  grow,  such  a  starfish  has  the 
appearance  of  a  comet,  or  body  with  a  long  tail.  In 
the  case  of  the  Ophiuroids,  it  is,  owing  to  the  cen- 
tralisation of  the  organs,  necessary  that  this  mode  of 
multiplication  should  be  effected  not  by  the  separa- 
tion of  a  single  arm,  but  by  the  division  of  the  disc, 
and  Ophiuroids  are  not  ^infrequently  to  be  seen  in 
which  there  are  three  shorter,  and  two  or  three  longer 
arms,  the  members  of  either  set  being  subequal  among 
themselves.  The  reproduction  of  arms  is  also  to 
be  frequently  noticed  among  the  Crinoids,  but  we 
have  not  yet  sufficient  information  as  to  whether  these 
arms  have  been  directly  broken  off  by  enemies,  or 
set  free  by  their  possessor  in  consequence  of  an  in- 
herited peculiarity,  as  observed  by  Jickeli  in  Antedon 
rosacea,  or  from  fear  of  danger.  This  kind  of  repro- 
duction has  not  been  observed,  as  may  well  be  sup- 
posed, among  Echinoids,  but  it  is  well  known  that 
Holothurians  will,  if  terrified,  eject  their  viscera,  and 
gradually  redevelop  what  one  must  suppose  to  be  im- 
portant organs  ;  the  Crinoids  will  replace  the  whole 
of  the  viscera  contained  in  their  calyx  (M.  Marshall). 

The  various  stages  of  reproduction  and  fusion 
which  are  to  be  observed  among  Echinoderms  can  be 
nearly  all  paralleled  by  what  is  to  be  seen  in  dif- 
ferent groups  of  the  animal  kingdom.  An  annelid  of 
the  southern  seas  (Palolo)  is  said  to  evacuate  its 
genital  products  by  completely  breaking  into  pieces, 
when  the  ova  and  spermatozoa  meet  and  unite  in  the 
water  ;  the  loss  of  a  single  arm  may  be  compared  to 
the  gradual  break-up  of  the  organism  which  is  seen  in 


494  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY, 

the  common  jellyfish  (Aurelia,  of.  page  541);  the  loss 
by  reflex  action  on  irritation  of  an  arm  is  paralleled  by 
the  tail  of  the  lizard  ;  while  the  formation  of  buds  by 
the  injured  arm  or  disc  reminds  us  of  the  capacity  for 
reproduction  which  is  exhibited  by  the  common 
Hydra. 

Asexual  reproduction  in  Echinoderms  may,  to 
sum  up,  be  one  of  two  things ;  there  may  be  fission 
with  repair,  or  a  single  arm  may  be  separated  off 
and  give  rise  by  external  gemmation  to  a  fresh  zooid. 

It  is  of  interest  to  note  that  these  phenomena  are 
seen  especially  in  those  forms  (Linckia)  in  which  the 
ossicles  of  the  arms  are  not  protected  either  by  strong- 
spines  as  in  Astropecten,  or  by  well-developed  margi- 
nal plates  as  in  Pentagonaster. 

A  high  grade  of  differentiation  is  reached  by  some 
of  the  Arthropod  a,  for  the  germinal  glands  are 
compact,  and  often,  indeed,  united  into  a  single  mass ; 
the  efferent  ducts  are  not  unfrequently  single,  and 
their  orifice  median  in  position,  while  the  distinction  of 
the  sexes  is  often  to  be  demonstrated  by  the  posses- 
sion of  secondary  characters,  such  as  differences  in 
external  configuration,  or  the  characters  of  certain 
pairs  of  appendages.  It  is  but  seldom  that  the  sexes 
are  united  in  the  same  individuals,  and  the  mode  of 
life  ordinarily  allows  us  to  find  an  easy  explanation 
for  this  arrangement,  when  it  does  obtain. 

On  the  other  hand,  there  are  some  remarkable  ar- 
rangements obtaining  in  various  Arthropods  which 
require  immediate  notice ;  in  the  Crustacean  Apus, 
and  in  such  insect  forms  as  the  bee  (Apis),  and  the 
plant-louse,  Aphis,  we  find  that  for  a  large  number  of 
generations  the  females  are  enabled  to  produce  ova, 
which  grow  up  to  the  adult  stage  without  the  inter- 
position of  any  male  influence;  here,  too,  as  in  the 
case  of  the  Rotifera  (see  page  482),  the  presence  of 
polar  globules  in  the  maturing  egg-cell  have  not  yet 


Chap.  XIII.]  GONADS    OF    CRAYFISH.  495 

quite  been  satisfactorily  demonstrated.  The  distinctions 
between  the  various  stages  of  what  was  originally  called 
parthenogenesis  will  be  considered  later  on. 

To  fix  our  ideas  of  the  peculiarities  of  the  Arthropod 
organisation,  we  may  commence  with  an  account  of  the 
generative  organs  of  the  crayfish ;  the  testes  and  ovary 
are  respectively  compact  glands  united  along  the 
middle  line,  and  giving  only  indications  of  a  primi- 
tively bilateral  origin;  they  occupy  a  definite  and 
constant  position  in  the  body,  lying  beneath  or  just 
in  front  of  the  heart,  and  having  the  enteric  canal  be- 
neath ;  they  both  give  off  a  duct  on  either  side,  which 
opens  to  the  exterior  at  the  base  of  one  of  the  paired 
appendages  which  are  connected  with  the  thorax  ;  as 
the  female  orifice  is  placed  two  segments  farther  for- 
ward than  the  male,  the  oviducts  are,  as  we  may  sup- 
pose, shorter  than  the  vasa  deferentia  which  carry 
away  the  products  of  the  testes  (Fig.  206). 

The  cells  which  line  the  walls  of  the  ducts  of  the 
testes  are,  at  the  ends  of  the  final  canals,  found  to 
occupy  swellings,  the  large  cells  of  which  undergo  that 
division  into  smaller  bodies  which  is  so  striking  a  charac- 
teristic of  spermatogenesis  \  the  products  of  these 
divisions  are  not,  however,  provided  with  merely  one 
protoplasmic  tail ;  in  the  Crustacea,  as  in  the  ISTema- 
tohelmmthes,  we  have  a  large  development  of  chitin 
in  various  parts  of  the  organism,  and  here,  as  there, 
we  have  spermatozoa  developed  which  are  without 
that  protoplasmic  flagellar  process,  which  is  to  be 
likened  to  a  cilium  ;  in  place  of  this  there  are  a  number 
of  stiff  processes,  the  disposition  of  which  has  gained 
for  the  spermatozoa  the  name  of  "radiate  cells."  In 
some  Decapoda  the  rays  retain  so  much  of  the  charac- 
ter of  the  pseudopodia  of  an  Amoeba  that  tfyey  can  be 
withdrawn  into  the  nucleated  portion  of  the  cell 
(Owsjannikoff). 

Immobile  cells  like  these  require  some  accessory 


496  COMPARATIVE  ANATOMY  AND' PHYSIOLOGY. 


organ  by  means  of  which  they  can  be  carried  with 

safety  to  the  body  of 
the  female,  and  here, 
as  in  so  many  of  their 
other  organs,  we  find 
the  crayfish  convert- 
ing some  of  its  ap- 
pendages into  a  suit- 
able apparatus  ;  the 
appendages  of  the 
first  two  segments  of 
the  abdomen,  that  is, 
of  the  two  segments 
which  lie  immedi- 
ately behind  that  on 
the  base  of  the  ap- 
pendages of  which 
are  placed  the  male 
orifices,  have  their 
terminal  portions 
converted  into  styli- 
form  processes,  with 
their  edges  so  folded 
on  themselves  as  to 
form  each  a  half- 
canal. 

In  the  ovary  we 
find  here,  as  in  so 
many  other  cases, 
that  one  cell  in  a 
special  set  (ovisac) 
grows  to  a  compara- 
tively large  size  at 
the  expense  of  those 
that  surround  and 
form  a  coat  for  it ; 

when  this  ovum  escapes  from  the  ruptured  ovisac  it 


Fig.  206.— Figures  of  the  Male  (A)  and  Fe- 
male (B)  Organs  of  Astacusfluviatilis. 

ov,  Ovary ;  od,  oviduct ;  od',  its  orifice  ;  t,  testis ; 
vd,  vas  deferens ;  vd',  its  orifice.  (After 
Huxley.) 


Chap.  XIII.]  GONADS   OF    COCKROACH.  497 

passes  into  the  oviduct,  where  it  is  perhaps  fertilised, 
and,  further,  provided  with  a  coat  (comparable  to  that 
by  means  of  which  the  spermatozoa  are  aggregated  into 
spermatophores),  one  end  of  which  is  drawn  out  into  a 
short  stalk  ;  by  means  of  this  stalk  the  developing  ova 
become  attached  to  the  small  appendages  of  the 
abdominal  region,  with  which  they  remain  connected 
till  they  are  converted  into  the  likeness  of  the  adult ; 
a  crayfish,  or  lobster,  at  this  stage  is  said  to  be  "in 
berry."  There  is,  then,  no  free-swimming  larval  stage 
in  the  fresh-water  crayfish. 

In  the  cockroach,  as  in  the  earthworm,  the  true 
character  of  the  testes  proper  has  been  misunderstood, 
owing  to  just  the  same  causes ;  it  is  in  young  males 
only  that  the  true  testes,  which  have  a  dorsal  position, 
can  be  detected ;  in  the  adult  forms  their  products  are 
found  in  the  reservoir  which  forms  the  double  head  of 
the  single  short  efferent  duct,  and  as  this  reservoir  is 
a  complicated  structure  (the  so-caRed  mushroom- 
shaped  gland),  formed  of  a  number  of  short  blind 
tubes,  within  which  the  spermatozoa  go  through  the 
later  stages  of  their  development,  it  has,  not  un- 
naturally, been  regarded  as  the  true  testis.  The 
matured  spermatozoa  are  thread-like  bodies  pointed  at 
either  end,  which  exhibit  a  wavy  movement. 

As  has  been  pointed  out  by  Waldeyer,  structures 
are  to  be  seen  in  the  ovaries  of  the  Arthropoda  which 
correspond  to  the  Graafian  follicles  of  the  Vertebrata 
(page  508). 

Gegenbaur  is  strongly  of  opinion  that  the  hiass 
of  generative  cells  in  the  Arthropoda  is  primitively 
single,  and  adduces  many  facts  in  support  of  this 
view  ;  not  only,  however,  is  this  arrangement  contrary 
to  that  which  obtains  in  all  other  bilaterally  sym- 
metrical animals,  but  it  is  further  opposed  by  certain 
embryological  facts ;  for  example,  the  Lepidoptera 
(butterflies  and  moths)  have,  in  the  later  stages  of 
G  G— 16 


498  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

embryonic  life,  an  organ  on  either  side  of  the  heart ; 
and,  lastly,  it  would  be  as  easy  to  derive  the  single 
from  the  double,  as  the  double  from  the  single 
arrangement,  when  we  bear  in  mind  that  so  primitive 
a  form  as  Peripatus  has  the  two  testes  completely 
separated  (Fig.  207). 


Fig.  207.— Male  Organs  of  Peripatus. 

sntia;  pr,  prostat 
(After  Balfour.) 


te,  Testes  ;  vd,  vasa  deferentia ;  pr,  prostates ;  p,  common  duct  of  v<L 
~;alfou    ' 


We  find,  then,  that  the  generative  glands  are 
either  distinctly  double,  united  by  an  obvious  bridge, 
or  converted  into  a  more  compact  single  mass,  which 
retains  more  or  less  obscurely  indications  of  a  primi- 
tively double  arrangement. 

The  male  glands  are  not  always  rounded  off  as  in 
the  crayfish  ;  in  Squilla  they  are  tubular,  and  from  the 
sides  of  the  walls  short  caeca,  in  which  the  generative 


Chap.  XIII.]  GONADS    OF   ARTHROPODA.  499 

epithelium  is  found,  are  given  off;  in  My  sis  the 
caeca  are  fewer  and  more  distinct,  and  in  Oniscus 
there  are  a  few  very  long  cseca. 

In  lulus  there  are  two  testicular  tubes  united  by  a 
number  of  median  branches,  and  provided  at  their 
sides  with  about  as  many  rounded  testicular  follicles. 
In  insects  the  testes  are  ordinarily  found  to  consist  of 
a  large  number  of  separate  tubes,  but  the  form  of  the 
compact  mass  varies  very  considerably,  and  no 
observations  seem  to  have  been  made  on  these  parts 
since  the  discovery  of  the  character  of  the  true  testi- 
cular organs  of  the  cockroach. 

While  the  spermatozoa  of  all  Crustacea,  with  the 
exception  of  the  parasitic  Cirripedia,  have  no  power  of 
independent  movement,  those  of  the  Insecta  are  wavy, 
and  one  end  is  often  rigid,  those  of  Myriopods  may  be 
rigid  or  motile,  and  those  of  the  Arachnida,  with 
which  Limulus  must  be  classified,  are  often  actively 
motile. 

The  hermaphroditic  arrangements  which  obtain  in 
the  Cirripedia  are  to  be  explained  by  their  fixed 
mode  of  life,  while  the  imperative  necessity  of 
avoiding  the  dangers  of  repeated  self -fertilisation  has 
in  some  cases  been  yielded  to  in  the  production  of 
minute  (^  inch)  and  degraded  males  (comple- 
mental  males)  (Darwin),  which,  as  in  the  case  of 
the  Gephyrean  Bonellia,  attach  themselves  to  the  body 
of  the  hermaphrodite,  or  simply  female  Cirriped. 

In  other  epizoic  Crustacean  parasites  (Achtheres 
percarum  and  other  Siphonostomata)  the  male  is  con- 
stantly smaller  than,  and  is  generally  found  attached 
to,  the  female  ;  here,  too,  as  in  the  case  of  the  Rotifera, 
the  number  of  males  is  much  smaller  than  that  of  the 
females,  and  adult  forms  are  often  developed  which 
arise  from  non-fertilised  ova. 

In  some  Isopoda  (e.g.  Cymothoa)  a  parasitic  habit 
likewise  obtains,  and  there  is  a  curious  mixture  of 


500  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

structural  and  functional  hermaphroditism ;  in  the 
younger  stages  the  testes  are  enormous  as  compared 
with  the  ovary,  and  two  penes  are  seen  to  be 
developed ;  the  spermatozoa  developed  in  the  glands 
of  these  Crustacea,  with  more  highly  differentiated 
ancestry  than  the  Cirripeds,  are,  it  is  said,  motionless. 
Later  on,  the  testes  diminish  in  size,  and  the  ovarian 
region  comes  into  functional  activity.  In  the  Cryp- 
toniscidse  the  male  elements  are  matured  during  the 
larval  stage,  and  male  free-swimming  larvse  copulate 
with  females  of  fixed  habit  and  a  less  high  degree  of 
organisation ;  the  male  larvae  subsequently  become 
degraded,  and  take  on  the  characters  and  develop  the 
glands  of  the  female  (Kossmann).  We  have  here  to 
do  with  what  may  be  called  phenomena  of  pro- 
tandrous  liermapliroditism. 

The  differences  between  males  and  females  are 
especially  well  marked  in  many  groups  of  Insects  ; 
as  an  ordinary  rule  the  males  are  smaller  than  the 
females,  being,  as  it  seems,  developed  more  rapidly  so 
as  to  be  ready  to  fertilise  their  often  short-lived  mate  ; 
where,  on  the  other  hand,  the  males  fight  with  one 
another,  or  carry  the  female  through  the  air,  they  are 
the  larger  of  the  two  sexes.  In  many  cases  (Cicadas, 
grasshoppers,  etc.)  the  males  are  alone  provided  with 
sound-producing  organs,  or,  as  so  often  happens 
among  butterflies,  the  males  are  much  handsomer 
in  appearance ;  sometimes,  also,  the  females  of  one 
species  are  of  two  distinct  forms  (dimorphic 
females),  and  among  the  beetles  we  find  that 
males  of  one  species  may  vary  very  greatly  in  the  size 
and  character  of  their  horns  (Lucanidse). 

Differences  in  size  obtain  also  among  some 
Arachnids  ;  the  male  spider,  for  example,  being  very 
much  smaller  than  the  female,  and  often  exceedingly 
agile  in  escaping  from  her  ferocity ;  in  the  spider,  as 
in  the  crayfish,  one  of  the  appendages  is  modified  to 


Chap.  XIII.]  GO  NADS    OF  MOLLUSCA.  $01 

serve  as  an  organ  for  conveying  the  sperm  to  the 
female. 

The  cephalous  Mollusca,  such  as  the  nvussel  or 
the  oyster,  retain  the  simple  conditions  of  generative 
glands,  being,  as  are  so  many  marine  forms  for  which 
the  water  serves,  by  its  currents,  as  the  carrier  of  the 
products  of  the  male  to  the  eggs  or  egg  receptacles  of 
the  female,  without  any  secondary  sexual  organs.  In 
general  character  and  appearance  also  the  male  glands 
closely  resemble  the  female,  and  it  is,  no  doubt,  in 
consequence  of  this  that  so  many  discussions  have 
arisen  as  to  the  monoecious  or  dioecious  arrange- 
ments of  certain  Lamellibranchs. 

As  seen  in  ordinary  cases,  the  glands  are  placed 
on  either  side  of  the  body,  and  each  has  a  separate 
orifice ;  with  a  continuous  outer  wall,  each  gland  is 
broken  up  into  a  number  of  separate  pouches,  and 
some  of  the  epithelial  cells  on  their  inner  face  become 
converted  into  ova  or  spermatozoa.  Small  at  most 
periods  of  the  year,  they  become  greatly  enlarged  at 
the  breeding  seasons,  when  they  occupy  a  large  part  of 
the  spaces  in  the  body  ;  the  ducts  are  ordinarily  short, 
and  the  ova,  on  escaping,  make  their  way  into  pouches 
in  the  gill  chambers,  where  they  are  fertilised  by  the 
spermatozoa  which  are  brought  in  by  the  currents  of 
the  water  of  respiration.  (See  page  221.) 

The  elaborate  investigations  of  Ryder  seem  to 
have  settled  the  problem  of  the  sexual  characters  of 
the  oyster ;  one  difficulty  in  the  determination  arises 
from  the  fact,  that  while  the  Portuguese  and  the 
ordinary  American  oyster  have  the  sexes  separate, 
the  common  edible  oyster  of  Europe  (Ostrea  edulis) 
has  the  sexes  united.  By  a  magnificent  effort  of 
histological  chemistry,  Ryder  has  shown  that  if  two 
colouring  matters  (safranin-red  and  methyl-green) 
are  brought  to  bear  on  suitably-prepared  sections  of 
the  body  of  an  edible  oyster,  the  red-staining  fluid 


502    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


affects  the  ova,  and  the  green  the  spermatozoa.  Before 
long  we  may  hope  to  see  this  method  of  investigation 
applied  to  other  problems  of  a  like  nature. 

In  the  Gastropoda  we  not  unfrequently  meet 
with  a  hermaphrodite  arrangement,  and  this  even 
among  the  lowest  forms ;  in  Proneomenia  Hubrecht 
has  observed  differences  of  colour  in  different  parts  of 
the  elongated  and  double  generative  gland ;  in  spirit 
specimens  the  light-yellow  portions  were  found  to  be 

ovarian,  and  the  brown- 
ish -  grey  parts  spermi- 
genous.  Here,  again,  we 
note  just  the  same  kind 
of  development  as  we 
have  seen  before;  the 
germinal  epithelium  gives 
rise  to  ova  in  one  region, 
and  spermatozoa  in  an- 
other. 

In  the  higher  Gastro- 
pods we  again  often  find, 
as  in  the  snail,  a  "  her- 
maphrodite gland," 
and  here,  too,  the  male  and  female  products  are  devel- 
oped in  different  portions  of  the  same  genital  area,  from 
cells  which  were  primitively  similar  in  character ;  the 
spermatoblasts  sometimes  become  free  from  the  wall 
at  an  early  stage  (Fig.  208),  and  in  some  cases  the  wall 
of  the  gland  is  produced  into  a  number  of  pouches. 
From  the  common  generative  gland,  or  ovotestis, 
there  leads  off  a  common  duct. 

In  those  Gastropods  that  are  not  hermaphrodite, 
and  in  the  Cephalopoda,  where,  too,  the  sexes  are 
separate,  there  is  very  generally  a  close  resemblance 
between  the  male  and  female  essential  organs,  remind- 
ing one  altogether  of  what  we  have  already  noticed  in 
the  Lamellibraiichiata. 


Fig.  208.  —Follicles  of  the  Ovotestis 

of  Helix  hortensis. 
oo,  Ova ;  ss,  spermatoblasts. 


Chap.  XIII]  GONADS    OF    CEPHALOPODA.  503 

As  in  the  Lamellibranchiata,  we  find  that  a  simpler 
arrangement  of  ducts  obtains  among  the  Gastropoda 
with  separate  sexes,  the  secondary  glands  and  copu- 
latory  organs,  which  are  so  well  developed  in  the 
monoecious  forms,  being  frequently  altogether  absent ; 
this  observation  does  not,  however,  apply  to  the 
Cephalopoda,  where  we  find  several  important  and 
instructive  complications. 

As  in  some  worms  and  nearly  all  Vertebrates,  the 
oviduct  is  not  directly  continuous  with  the  proper 
wall  of  the  ovary,  but  the  ova  are  set  free  into  the 
cavity  of  the  capsule  which  encloses  the  ovary,  to 
be  thence  taken  up  by  the  open  mouth  of  the 
oviduct.  This  is  either  single  or  double,  and  has  a 
considerable  extent,  or  only  the  terminal  portion,  of 
its  walls  provided  with  secreting  glands.  Near  the 
orifice  of  the  oviduct  there  open  the  ducts  of  two 
large  glands,  which  lie  on  the  branchial  cavity,  and 
which  secrete  a  viscous  substance,  by  means  of  which 
the  ova  are  massed  into  groups ;  these  are  the  so-called 
nidamental  glands. 

The  vas  deferens  or  duct  from  the  testis,  which 
may  or  may  not  be  double,  is,  like  the  oviduct,  not 
directly  continuous  with  the  gonad ;  it  is  considerably 
coiled,  and  glands  or  pouches  are  developed  along  its 
tract ;  of  these  the  most  important  is  that  which  is 
ordinarily  known  as  IVeedham's  pouch,  in  which 
are  collected  the  masses  of  spermatozoa  that  have  been 
grouped  together  on  their  way  down  the  duct  (Fig.  209). 

These  spermatophores  are  tubular  structures,  which 
may  be  about  half  an  inch  in  length  ;  the  spermatozoa 
are  grouped  together,  and  the  rest  of  the  cavity  of  the 
tube  is  occupied  by  a  coiled  body  ;  when  the  sperma- 
tophore  escapes  by  the  penis  into  the  water  the  ex- 
ternal sheath  becomes  ruptured,  and  the  coiled  elastic 
band  within  being  set  free  forces  the  sac  of  sperma- 
tozoa out  of  the  containing  sheath  (Fig.  210). 


504  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Though  the  germ  glands  of  the  higher  Mollusca 
are  simple  and  similar  in 
structure,  there  is  in  some 
a  very  complex  system  of 
accessory  organs.  In  the 
hermaphrodite  forms  the 
duct  remains  common  for 
a  short  distance  only,  and 
its  tract  becomes  compli- 
cated by  the  development 
of  glandular  bodies,  the  se- 
cretion of  which  nourishes 
or  protects  the  ova,  and  of 
pouches,  in  which  the  sperm 
received  during  copulation 
can  be  stored  up  till  the 
ova  are  ready  for  fertilisa- 
tion. The  male  portion  has 
connected  with  it  glands, 
by  means  of  which  the 
spermatozoa  are  massed  into 
spermatophores,  and 
the  integument  is  invagi- 
natedto  form  a  penis,  which, 
when  turned  inside  out, 
forms  a  duct  for  the  sperm. 
In  some  cases  also,  as  in 
the  snail,  each  individual 
is  provided  with  a  gland 
which  secretes  a  chitinous 
Pig.  209.-  Male  jDuct  of  Loiigo  dart-like  body  (dart  sac), 

«,  Penis  ;^pouITLdham;c,vas     which    is    thrown    off  from 

e:)te     each 


,er  snail    at    its    mate 

ct)0  capsule   in   the   Preliminary  stages 

of  copulation. 
In  some  Cephalopods  there  is  a  still  more  remark- 
able provision  for  the  safety  of  the  seminal  products. 


Chap.  XIII.] 


HECTOCOTYLUS. 


505 


Within  the  mantle  cavity  of  a  female  Argonaut,  Cuvier 

discovered  an  elongated  worm-like  body,  provided  with 

a  number  of  suckers.     This  he  regarded  as  a  parasite, 

and  in   consequence  of  its  appearance  conferred  on  it 

the  distinctive  name  of  Hectocotylus  Argonautse. 

This  "  ver  bien  extraordinaire,"  as  Cuvier  called  it,  is 

nothing  more  than  one  of  the  arms 

of  the  male  Argonaut,  which  has 

undergone  a  remarkable  alteration, 

become  greatly  elongated,  and  had 

conveyed  within  it  the  spermato- 

phore.     When  fully  developed  it 

breaks  away  from  the  individual 

that  has  produced  it  and  makes 

its  way  into  the  mantle  cavity  of 

the  female. 

There    are    various  stages   of 

"  hectocotylisation  "    intermediate 

between  this   extreme   form   and 

the  much  more  simple  extrusion  of 
the  spermatophore  which  obtains 
in  Loligo  ;  in  the  latter  the  fourth 
arm  on  the  left  side  has  its  suckers 
rudimentary ;  in  Octopus  it  is 
the  third  on  the  right  side,  and 
in  it  the  free  end  is  provided  with 
a  spoon-shaped  plate,  to  which 
the  spermatophore  is  conveyed. 

The  general  lessons  as  to  the 
primitive  origin  of  germinal  cells  may  be  well  impressed 
on  the  mind  by  a  consideration  of  what  obtains  in  the 
Craniate  Chordata.  Among  the  epithelial  cells 
which  line  the  body  cavity,  some  become,  at  an  early 
stage,  sharply  distinguished  from  the  rest,  but  of  these 
germinal  cells  it  is  not  at  first  possible  to  say 
which  are  male  and  which  female;  later  on  they 
undergo  their  characteristic  changes,  the  male  cells 


Fig.    210.  —  Spermato- 
phore of  Loligo  pealii. 

a,  Sheath  ;  b,  spermatozoa ; 
c,  coiled  band.  (After 
W.  K.  Brooks.) 


506  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

gradually  breaking  up,  and  the  female  becoming  sur- 
rounded with  follicular  cells  and  growing  at  their 
expense.  The  epithelial  cells  that  are  about  to  be- 
come germinal  are,  in  the  early  stages  of  both  sexes, 
formed  on  a  genital  ridge,  or  "  sexual  eminence " ; 
but,  while  this  ridge  becomes  more  prominent  in  the 
female,  it  gradually  disappears  in  the  male.  In 
other  words,  the  distinction  between  the  two  sexes 
is,  at  this  point,  well  marked  at  a  very  early  period. 

With  the  exception  of  Amphioxus,  no  Chordate 
presents  any  distinct  indications  of  the  metameric 
segmentation  of  its  reproductive  organs,  and  the 
consolidation  which  was  observed  in  so  many  of 
the  non-vertebrated  forms  is  just  as  well  marked 
in  this  phylum.  The  higher  the  vertebrate,  the 
more  compact  its  genital  glands. 

In  Petromyzon  the  testis  extends  almost  through- 
out the  whole  length  of  the  body  cavity ;  in  the 
Elasinobranchs  it  is  best  developed  in  the  anterior 
region  of  the  body;  and  in  most  Ganoids  it  is 
rather  smaller  than  in  Elasmobranchs ;  in  the 
Teleostei  the  glands  are  known  to  vary  much  in 
size  at  different  times  of  the  year,  but  they  are 
generally  of  considerable  length.  In  the  Csecilise 
the  testes  are  broken  up  into  a  number  (about  ten) 
of  separate  parts,  connected  one  with  another  by 
ducts,  and  having,  on  either  side,  the  appearance  of  a 
string  of  pearls.  In  the  Urodeles  and  Anura  the 
testes  are  more  or  less  elongated  or  rounded,  but 
never  form,  externally,  more  than  a  single  mass.  As 
we  pass  through  Keptiles  to  Birds  we  observe  the  same 
phenomenon,  though  it  is  necessary  to  note  that  the 
size  of  the  organs  varies  very  much  with  the  conditions 
of  virile  activity  \  for  example,  the  testes  of  the 
sparrow,  which  in  January  are  only  two  millimetres 
long,  are  in  April  15  mm.  long,  and  wider  in  pro- 
portion. 


Chap.  XIII.]  GONADS   OF  MAMMALIA.  507 

The  next  important  step  in  the  differentiation  of 
the  testes  is  the  great  change  in  their  topographical 
relations  which  is  to  be  observed  in  the  higher 
Mammalia,  a  change  which  is  of  the  more  signifi- 
cance, as  it  is  one  that  is  never  suffered  by  the  ova- 
ries. In  the  Monotremata  the  testes  depart  but  little 
from  their  primitive  position  ;  in  the  porpoises  they 
lie  beneath  the  kidney  ;  in  the  hedgehog  they  never 
leave  the  abdomen,  though  they  descend  as  far  as  the 
inguinal  ring  ;  *  in  the  horse  they  pass  through  the 
ring,  which,  however,  remains  permanently  open,  and 
they  make  their  way  into  a  sack  which  is  formed  by 
the  integument,  and  is  known  as  the  scrotum.  In 
man,  finally,  as  in  some  other  forms,  the  testes  are 
permanently  enclosed  in  a  scrotum,  although  in  indi- 
vidual cases  of  arrested  development  we  have  the 
atavistic  arrangement  of  the  testes  being  throughout 
life  abdominal  in  position. 

The  ovaries,  like  the  testes,  become  more  compact 
as  the  scale  of  vertebrate  organisation  is  ascended.  In 
both  sexes  we  may  find  that  the  gonad  of  one  side 
projects  farther  forwards  and  that  of  the  other  farther 
back. 

In  rare  cases,  and  in  individual  or  specific  varia- 
tions (as  in  the  toad,  cod,  herring),  rudimentary  testes 
are  to  be  observed  in  females,  and  rudimentary  ova- 
ries in  males.  On  the  other  hand,  Serranus,  among 
fishes,  would  appear  to  be  ordinarily  hermaphrodite, 
and  in  individual  cases  only  to  have  the  sexes 
separate. 

The  spermatozoa  of  Elasmobranchs  have,  as 
a  rule,  larger  heads  than  those  of  Teleostean  fishes ; 
in  the  triton,  salamander,  and  some  other  Amphibia  a 
remarkable  undulating  membrane  is  developed  on  the 

*  The  inguinal  ring  is  the  inferior  or  cutaneous  orifice  of  the 
inguinal  canal,  or  space  in  the  abdominal  wall  by  which  the 
spermatic  cord  passes  to  the  testis. 


508  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

tail  of  the  spermatozoon ;  in  some  birds  the  head  is 
not  straight,  but  is  twisted  into  a  spiral  (Canary). 
In  the  Mammalia  there  is  nearly  always  a  rounded 
head  and  a  long  tail. 

The  ova  of  Elasmobranchii  and  Teleostei,  Am- 
phibia and  Sauropsida,  are  always  of  some  size,  the 
first  and  the  last  of  these  groups  having  much  the 
largest  ova,  or  eggs  most  richly  provided  with  food 
yolk.  In  Amphioxus  and  in  mammals  *  the  eggs 
are  very  much  smaller,  and,  in  consequence  of  the 
absence  of  the  disturbing  element  (the  food  yolk)  the 
early  stages  of  development  are  very  different.  In 
addition  to  the  investing  membranes,  which  are  some- 
times perforated  by  a  micropyle,  some  eggs  are  pro- 
vided with  a  calcareous  coating  or  shell. 

Calling  to  mind  somewhat  the  arrangements  which 
obtain  in  the  Arthropoda,  the  eggs  of  vertebrates  are 
enclosed  in  an  envelope  of  cells  which  owe  their 
origin  to  the  primitive  germinal  epithelium  ;  in  the 
Mammalia  this  cellular  investment  becomes  more  than 
one  layer  deep,  the  cavity  within  enlarges,  and  the 
ovum  projects  into  the  fluid  therein  contained  ;  this  is 
the  structure  which  is  ordinarily  known  as  the 
Oraafian  follicle. 

The  numbers  of  ova  formed  within  any  one  female 
vary  enormously,  and,  as  may  be  supposed,  the 
largest  numbers  are  found  in  the  lowest  forms,  or,  in 
fishes  ;  thus  the  cod  is  credited  with  nine  millions, 
the  haddock  and  plaice  with  six  millions  ;  the  Elasmo- 
branchs  have  a  very  much  smaller  number,  often  less 
than  ten  ;  in  the  Amphibia  there  are  often  a  consider- 
able number,  in  the  Sauropsida  the  number  is  smaller, 
and  no  mammal  has  more  than  about  ten  matured  at 
the  same  time,  while  in  many  cases  the  birth  of  twins 
even  is  only  an  occasional  phenomenon. 

These   ova   become   invested    in    membranes,    of 
*  The  Monotremata  have  been  lately  stated  to  have  large  eggs. 


Chap,  xiii.]  AMNION;   ALLANTOIS.  509 

which  the  most  common  are  the  vitelline  and  the 
zona  radiata  ;  in  some  cases  a  third,  more  internally 
placed  membrane,  is  also  developed.  In  addition  to 
these,  there  appear  in  the  Sauropsida  and  Mammalia 
two  very  important  foetal  membranes ;  the  first  of 
these  forms  an  enveloping  and  protecting  sac  for  the 
embryo,  and  is  called  the  amiiion;  it  owes  its 
origin  to  two  folds,  which  appear  one  at  either  end  of 
the  embryo,  rise  up,  curve  over,  and  finally  unite  in 
the  middle  line  above.  As  each  fold  is  double  it 
follows  that,  when  the  ends  of  each  unite,  two 
envelopes  are  formed,  and  a  cavity  or  space,  which 
later  on  becomes  filled  with  the  liquor  amnii,  is 
developed  between  them.  The  inner  of  these  envelopes 
forms  the  true  aiiiuion,  and  the  outer  the  false 
amnion  or  subzonal  membrane  (Fig.  211)  . 

The  other  covering  is  the  allantois,  and  it  has  a 
very  different  history.  Early  in  the  development  of 
the  intestine  there  is  given  off  from  in  front  of  the 
region  of  the  future  anus  a  saccular  outgrowth,  which 
gradually  extends  beyond  the  body  of  the  embryo, 
and  over  the  greater  part  of  it ;  during  the  period  of 
foetal  development  this  allantois  remains  connected  by 
a  stalk  with  the  body  of  the  embryo,  and  over  its 
surface  there  extends  a  rich  supply  of  blood-vessels, 
the  allantoic  arteries  and  veins ;  so  that  it  has  a 
nutrient  or  a  respiratory  function,  or  both. 

In  the  Sauropsida  all  the  material  for  the  develop- 
ment of  the  embryo  is  contained  within  the  egg  itself, 
which,  at  a  very  early  period,  becomes  sharply  divisible 
into  a  blastoderm,  the  seat  of  the  future  essential 
stages  in  development,  and  a  yolk  sac,  or  store  of 
nutriment,  the  material  from  which  is  acquired  by  the 
embryo  through  the  intermediation  of  the  rich  plexus 
of  vessels  which  are  rapidly  developed  within  and 
around  it.  As  the  yolk  sac  diminishes,  the  allantois, 
for  a  time,  increases  in  size,  its  vessels  attain  to 


/jj 


Fig.  211.— Diagrams  to  show  the  Development  of  the  Atnnion  and 
Allantois.     (After  Foster  and  Balfour. ) 

In  A  the  folds  of  the  amnion  (at)  are  to  be  seen  rising  up  on  either  end  of  the 
embryo  (e) ;  in  B  they  have  nearly  met,  and  in  c  they  have  entirely  coalesced. 
In  D  the  allantois  («0  is  seen  making  its  way  out  of  the  body  of  the  embryo, 
and  in  E  it  is  seen  at  a  later  stage.  In  v  the  yolk  sac  is  shown  in  position, 
and  in  G  just  before  its  final  absorption. 


Chap,  xiii.]       OVIDUCTS  OF  VERTEBRATES.  511 

greater  physiological  importance,  and  give  to  this  out- 
growth the  function  of  a  respiratory  organ. 

The  fate  and  functions  of  the  membranes  of  the 
mammalian  ovum  will  be  considered  a  little  later 
(page  515). 

In  the  lower  Vertebrates  there  are  no  special 
efferent  ducts  for  the  genital  products ;  as  in  the 
Chsetognatha,  these  fall  into  the  coelom,  and  make 
their  way  into  the  surrounding  water  through  the  two 
abdominal  pores ;  in  the  Cyclostomata  these  pores 
represent,  no  doubt,  the  primitive  mode  of  exit  of  the 
ova  and  spermatozoa,  but  in  the  higher  Teleostei, 
where,  again,  they  (or  parts  called  by  the  same  name) 
have  the  same  function,  it  is  to  be  distinctly  borne  in 
mind  that  proper  ducts  have  been  there  developed, 
and  have  subsequently  undergone  atrophy. 

The  arrangement  of  oviducts  which  obtains  in  the 
frog  is  that  which  is  most  common,  indeed  almost 
universal,  among  Vertebrates ;  as  in  the  earthworm, 
the  ova  escapes  from  the  ovary  into  the  body  cavity, 
whence  they  are  taken  up  by  the  open  mor  '*  of  the 
paired  oviducts.  These  two  mouths  are  sometimes 
(Elasmobranchs,  most  Ganoids)  fused  into  a  common 
ostiuin,  but  the  greater  part  of  the  ducts  are 
distinctly  separated  from  one  another.  As  has  been 
already  pointed  out  (page  262)  these  ducts  (Mullerian 
ducts)  are  derived  from  the  primitive  excretory  duct. 

In  L<epidosiren  Hyrtl  has  described  the  ab- 
dominal orifice  of  each  duct  as  being  funnel-shaped, 
and  this  form  of  opening  is  commonly  found  in  all  the 
pentadactyle  Vertebrata.  As  we  ascend  the  scale  we 
find  the  oviducts  becoming  more  compact,  in  so  far 
that  the  orifice,  which  in  the  Amphibia  and  in  the 
lizard  (Fig.  212;  ot)  lies  far  forward,  and  in  front  of 
the  ovaries,  becomes  in  snakes  and  mammals  set 
much  farther  back ;  in  the  latter  the  oviducts  are 
often  spoken  of  as  the  Fallopian  tubes. 


Ot 


-..Ob 


,  Kidney;  B,  blad- 
der ;  B',  neck  of 
bladder ;  B,  rec- 
tum ;  cl,  cloaca ; 
K',  rectal  open- 
ing into  cloaca  ; 
ur',  opening  of 
ureter  into 
cloaca;  ov,  ov- 
ary ;  ot.  opening 
of  oviduct;  od, 
oviduct  ;  od',  its 
opening  into  tlie 
cloaca".  (After 
Wietlerslieim.) 


mm 

Fig.  212.— Urogeuital  Organs  of  the  Lizard  (Lacerta  mumlis). 


Chap,  xiii.]  OVIDUCTS.  513 

In  the  Teleostei,  and  the  teleostean-like  Ganoids, 
such  as  Lepidosteus,  the  oviducts  have  no  patent 
abdominal  mouths,  but  the  walls  of  the  tubes  are, 
continuous  with  those  of  the  gland,  or  the  ducts 
become  more  or  less  aborted ;  here  it  would  appear 
that  the  oviduct  is  not  the  altered  Mullerian  duct,  but 
sufficient  information  on  this  head  remains  to  be 
acquired.  When,  as  in  eels  and  salmons,  the  ducts 
become  completely  aborted,  the  ova  fall  into  the  body 
cavity  and  escape  to  the  exterior  by  the  so-called 
abdominal  pores. 

The  different  regions  of  the  oviducal  canal  take  on 
very  various  functions  in  various  vertebrates;  one 
important  factor  in  determining  the  character  of  the 
ducts  is  the  size  of  the  ova  which  pass  through  it,  and 
another  is,  of  course,  to  be  found  in  the  different 
conditions  of  size  and  age  in  which  eggs  are  laid  or 
young  brought  forth.  In  the  frog  the  whole  of  the 
canal,  with  the  exception  of  the  penultimate  portion, 
is  of  the  same  calibre  throughout ;  in  the  terminal 
enlargement  the  ova  become  grouped  into  masses, 
which  are  held  together  by  the  gelatinous  substance 
secreted  by  the  oviducal  glands  ;  the  ova  are  fertilised 
outside  the  body  of  the  female.  In  some  Elasmo- 
branchs,  and  in  the  Sauropsida,  the  ova  as  they  pass 
down  the  ducts  become  not  only  invested  by  a  layer 
of  albumen,  but  surrounded  by  a  shell ;  this  may  be 
horny,  as  in  Elasmobranchs,  Lacertilia,  and  Ophidia  ; 
or  firmer  and  calcareous,  as  in  other  Reptiles  and  in 
Birds. 

In  the  Ophidia  the  right  ovary  is  always  larger, 
and  often  much  larger,  than  the  left,  and  the  left 
oviduct  shorter  than  the  right ;  in  Birds,  on  the  other 
hand,  the  right  ovary  and  duct  become  aborted  during 
development,  though  indications  of  their  presence  are, 
of  course,  retained  by  some  forms. 

Where  the  ova  are  not  only  fertilised  within  the 
HH— 16 


514  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

body  of  the  female,  but  also  pass  through  the  early 
stages  of  development  within  it,  the  ducts  become 
much  more  elaborate  than  in.  the  oviparous  forms  ; 
Vertebrates  in  which  this  obtains  are,  somewhat  im- 
properly, spoken  of  as  viviparous.  The  most  pro- 
minent phenomenon  is  the  enlargement  of  the  distal 
portion  of  the  duct  into  a  cavity  of  varying  width,  the 
so-called  uterus.  In  most  of  the  viviparous  fishes, 
in  the  few  viviparous  Lacertilia  (Zootoca,  Anguis 
fragilis),  and  in  the  viviparous  Ophidia,  the  ova  appear 
to  continue  to  develop  within  the  uterine  cavity  with- 
out any  assistance  or  nourishment  from  the  mother. 

In  a  few  Elasmobranchs,  however,  and  in  all  the 
higher  Mammalia,  means  are  taken  by  which  the 
young  are  brought  into  real  relation  with  the  parent, 
arid  are  directly  nourished  by  its  blood ;  in  the 
former  (e.g.  Mustelus  Isevis)  the  vascular  walls 
of  the  yolk  sac  become  raised  into  ridges  which 
fit  into  corresponding  depressions  in  the  vascular 
uterine  membrane.  Here  we  have  the  lowest  and 
simplest  representation  of  a  placenta. 

Of  the  aplacental  Mammals  little  is  definitely 
known,  and  what  information  we  have  was  long  ago 
furnished  by  Owen,  who  observed  in  Ornithorhynchus 
that  the  ova  lay  free  in  the  uterus,  the  lining 
membrane  of  which  was  highly  vascular  ;  in  a  foetal 
kangaroo  there  were  folds  on  the  investing  membrane 
of  the  foetus,  and  corresponding  depressions  on  the 
walls  of  the  uterus,  but  no  organic  connection  was 
observed  between  the  parent  and  the  young. 

In  the  placental  Mammals  outgrowths  of  the 
investing  membranes  of  the  egg  become  more  or  less 
closely  united  with  vascular  outgrowths  of  that 
portion  of  the  oviduct  which  has  the  function  of  a 
uterus,  and  give  rise  to  a  characteristic  organ. 

To  make  such  structures  intelligible  it  is  necessary, 
first  of  all,  to  give  some  account  of  the  characters  of 


Chap,  xni.]       PLACENTA  OF  MAMMALS.  .  515 

these  membranes ;  it  will  be  remembered  that,  in 
speaking  of  the  extra-uterine  development  of  the 
embryo  of  the  Sauropsida  (the  other  division  of 
the  Amniota,  as  the  Mammalia  and  Sauropsida  are 
often  collectively  called)  attention  was  directed  to 
the  amnion,  the  allantois,  and  the  yolk  sac.  As  in 
the  other  division,  the  embryo,  notwithstanding  the 
smallness  of  the  mammalian  egg,  is  early  distinguished 
into  an  embryonic  and  a  vitelline  portion,  but  the 
yolk  sac  *  is  always  smaller  than  in  Sauroids  ;  the 
false  amnion,  or  subzonal  membrane,  has,  moreover, 
an  important  part  to  play,  which,  from  the  nature  of 
things,  was  impossible  in  the  oviparous  groups,  and 
the  yolk  sac  has,  in  some  mammals,  a  certain  re- 
lation to  the  uterine  walls. 

Dealing  first  with  such  a  case  (e.g.  the  rabbit) 
we  find  the  developing  embryo  soon  becoming  attached 
to  and  embraced  by  the  epithelium  which  lines  the 
uterus ',  the  epiblastic  covering  of  the  yolk  sac 
separates  from  the  subjacent  layers  and  unites  with 
•  the  false  amnion  to  form  a  layer  beneath  the  zona 
radiata  (page  509) ;  a  little  later  the  latter  and  the 
subzonal  membrane  fuse  together.  Later  on, 
this  subzonal  membrane  fuses  with  the  vascular  walls 
of  the  yolk  sac,  to  form  a  fresh  investing  membrane, 
the  false  chorion.  The  true  chorion  is  formed 
by  the  fusion  of  part  'of  the  vascular  wall  of  the 
allantois  with  part  of  the  subzonal  membrane. 
Into  the  substance  of  this  chorion  there  extend  blood- 
vessels, and  from  its  surface  there  grow  out  a  number 
of  delicate  processes  which  fit  into  corresponding 
vascular  "  crypts  "  which  are  developed  on  the  inner 
wall  of  the  uterus ;  the  whole  of  the  apparatus  thus 
formed,  partly  by  the  mother  and  partly  by  the  foetus, 
is  known  as  the  placenta. 

*  In  mammals  the  yolk  sac  is  generally  known  as  the  "um- 
bilical vesicle." 


516  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

As  was  first  pointed  out  by  Balfour,  the  types  of 
mammalian  placentae  fall  primarily  into  two  great 
groups ;  in  one  the  yolk  sac  (or  so-called  umbilical 
vesicle)  takes  a  share  in  the  formation  of  the  placenta  ; 
this  arrangement,  which  is  probably  the  more  primi- 
tive, is  retained  by  the  Insectivora,  Rodentia,  and 
Chiroptera.  The  other  type,  which  is  seen  in  all  the 
other  forms,  is  characterised  by  the  fact  that  the  yolk 
sac  ceases  to  take  any  important  share  in  the  nourish- 
ment of  the  foetus. 

Placentas,  in  which  the  foetal  portion  is  derived 
chiefly  from  the  allantois,  and  not  from  the  yolk  sac, 
fall  again  into  two  great  divisions  ;  in  the  simpler 
forms  the  outgrowths  (villi)  of  the  chorion  merely 
project  into  the  pits  which  are  developed  in  the  uterus 
of  the  mother,  and  may,  at  birth,  be  drawn  out  from 
them  without  injuring  the  uterine  vessels ;  these  are 
the  non-deciduate  placentas.  In  the  other  the 
connection  between  the  walls  of  the  villi  and  those  of 
the  pits  becomes  so  intimate  that  the  vessels  of  the 
uterus  are  torn  when  the  foetus  is  born;  such  pla- 
centaa  are  called  deciduate.  The  former,  again, 
belong  to  one  of  two  groups  ;  in  the  horse  or  the  pig 
the  chorionic  villi  extend  over  nearly  the  whole  of 
the  surface  of  the  placenta,  and  we  have  a  diffused 
placenta  ;  in  others,  such  as  the  sheep  or  the  cow, 
the  villi  are  collected  into  definite  tufts  or  branches, 
which  are  distinctively  known  as  cotyledons,  and 
the  placenta  is  said  to  be  cotyledonary.  The  spaces 
between  the  cotyledons,  of  which  the  cow  and  the 
goat  have  from  sixty  to  one  hundred,  but  the  deer 
only  from  eight  to  twelve,  are  left  bare  of  villi. 

In  the  deciduata  the  placenta  is  ordinarily  said  to 
be  discoidal  or  zonary,  but  care  must  be  taken  in 
the  application  of  the  former  of  these  two  terms,  for 
the  placenta  is  disc-shaped  in  insectivores,  rodents,  and 
bats,  as  well  as  in  man  and  the  apes  ;  it  is  convenient, 


Chap,  xiii.]       PLACENTA  OF  MAMMALS.  517 

therefore,  to  follow  Balfour  in  applying  the  term 
metadiscoidal  to  the  disc-shaped  placentae  of  such 
forms  as  have  the  placenta  formed  by  the  allantois, 
rather  than  by  the  yolk  sac.  From  the  diffused  or 
discoidal  forms,  or  from  both,  there  has  been  evolved 
another  form  of  placenta,  such  as  is  seen  in  the  Car- 
nivora  and  some  other  mammals ;  in  these  the  villi 
are  confined  to  a  broad  girdle-shaped  region  of  the 
chorion,  and  we  have  therefore  an  arrangement  which 
may  well  be  spoken  of  as  zonary. 

The  oviducts  of  all  vertebrates  are  typically 
paired,  and  in  many  cases,  as  in  the  Amphibia,  they 
are  completely  separate  from  one  another  along  the 
whole  of  their  course,  as  they  are  also  in  the  Reptilia, 
although  in  the  Chelonia  their  terminal  portion  is  in- 
vested in  a  common  sheath. 

In  Teleostei  and  Elasmobranchs  the  two  ducts 
unite  at  their  termination  to  open  by  a  common  orifice; 
in  the  Teleostei  they  open  to  the  exterior  behind  the 
anus ;  in  Elasmobranchs,  Dipnoi,  and  all  the  Sauro- 
psida,  they  open  into  a  pit  or  cloaca,  which  is  common 
to  them,  the  renal,  and  the  rectal  orifices. 

In  the  Monotremata,  in  which  there  is  a  distinct, 
and  the  Marsupialia,  in  which  there  is  a  shallow, 
cloaca,  the  oviducts  open  separately ;  in  the  latter, 
but  not  in  the  former,  the  lower  part  of  the  oviducal 
tube  is  modified  to  form  a  vagina,  which,  however 
modified  in  various  forms,  is  essentially  double.  In 
the  higher  Mammals,  in  which  the  common  urino- 
genital  orifice  is  placed  in  front  of  the  anus,  and  sepa- 
rated from  it  by  a  more  or  less  distinct  perinseiiui, 
the  terminal  portions  of  the  two  oviducts  are  always, 
at  least  externally,  and  in  most  cases  also  internally, 
single  and  united.  As  we  ascend  the  soale  of  the 
Eufcheria  we  find  this  confluence  becoming  more  and 
more  extensive.  In  the  great  ant-eater  there  is  a 
remnant  of  the  two  walls  of  the  separate  tubes  in 


518  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  form  of  a  longitudinal  septum,  which  is  about 
an  inch  in  length  and  terminal  in  position,  so  that 
the  vaginal  tube  opens  by  two  separate  orifices  into 
the  urino-genital  canal.  In  some  examples  of,  though 
apparently  not  in  all,  Indian  elephants,  the  vagina 
and  the  uterus  above  are  divided  into  two  by  a  still 


Fig.  213.— Diagrams  to  Show  the  Different  Stages  of  the  Mammalian 
Uterus. 

A,  Uterus  duplex  ;  B,  uterus  bipartitus;  c,  uterus  Incornis  ;  n,  uterus  simplex  ; 
od,  oviduct ;  ut,  uterus ;  \g,  vagina.    (After  Wiedersueiin.) 

more  complete  longitudinal  septum  ;  in  the  rodent 
Lagostomus  the  septum  is  retained  in  the  proximal 
or  uterine,  but  is  absent  from  the  distal,  portion  of 
the  vagina.  As  an  occasional  occurrence,  a  "  vagina 
duplex  "  is  found  in  the  human  subject. 

In  those  Mammals  in  which  the  vagina  is  com- 
pletely single  we  find  (1)  two  quite  distinct  uteri, 
each  opening  by  its  own  os  into  the  vagina  (Fig.  213  ; 
A)  ;  this  condition  obtains  in  the  rabbit ;  (2)  the 
undivided  portion  of  the  uterus  is  short,  as  in  the 


Chap,  xiii.]  MALE  DUCTS.  519 

hedgehog,  and  the  remaining  portions  of  the  ducts,  or 
so-called  cormia  iiteri,  are  long ;  (3)  in  the  mare 
the  cornua  are  proportionately  shorter ;  and  (4)  in 
the  highest  forms,  as  in  man,  the  short  proximal 
portions  of  the  oviducts  ("Fallopian  tubes")  open 
directly  into  the  large  single  uterus. 

As  has  already  been  pointed  out  (see  page  263), 
the  male  have  a  different  origin  to  the  female 
generative  ducts  of  Vertebrates,  arising,  as  they  do, 
from  the  primitive  Wolffian  duct.  On  the  whole 
they  present  less  important  points  of  difference  in 
various  groups  than  do  the  oviducts  ;  and  they  con- 
stantly remain  separate  along  the  whole  of  their 
course  ;  where  there  is  a  terminal  portion  which  ap- 
pears to  be  single,  it  is  not  part  of  the  Wolffian  duct. 

In  the  ichthyoid  vertebrates  the  seminal  products 
always  make  their  way  through  the  kidney ;  in  the 
lizard  the  testicular  duct  (vas  defereiis)  and  the 
ureter  only  unite  just  before  they  open  into  the  cloaca. 
In  Birds  the  two  ducts  run  close  to,  but  open  sepa- 
rately from,  one  another.  In  the  Monotremata  the 
renal  and  seminal  ducts  open  separately  into  the 
cloaca,  and  the  common  urine-genital  passage  is  distal 
to  it. 

In  the  remaining  Mammalia,  where  the  ureters 
open  separately  into  the  bladder,  there  is,  again,  no 
relation  between  them  and  the  Wolffian  ducts  ;  in 
place  thereof,  however,  we  find  -a  common  passage, 
the  vasa  deferentia,  opening  at  some  point  on  the 
course  of  the  urethra,  which  is  itself  the  narrowest 
proximal  end  of  the  allantois. 

Copulation  is  not  always  effected  among  fishes, 
some  only  discharging  their  genital  products  into  the 
water ;  in  the  Elasmobranchs  the  hinder  pair  of  fins 
serve  as  organs  for  dilating  the  female  orifice,  and 
between  them  there  is  placed  a  papilla,  on  which 
the  seminal  ducts  open,  and  this  is  inserted  into 


520    COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

the  female.  Union  of  the  sexes  is  common  among 
the  Amphibia,  but  it  is  in  the  Csecilise  only  that 
an  external  copulatory  organ  is  developed  from  the 
cloaca ;  this  may  be  as  much  as  five  centimetres 
long  (Wiedersheim).  In  Reptiles  the  penis  is  nearly 
always  distinctly  double  ;  in  the  lizard  the  two  halves 
ordinarily  lie  beneath  the  skin  just  behind  the 
cloacal  slit,  but  they  are  here,  as  they  are  also  in  the 
snakes,  capable  of  protrusion  and  erection  ;  a  spiral 
groove  runs  along  the  inner  face  of  each  half,  and 
serves  as  the  duct  by  which  the  semen  is  conveyed 
from  the  vas  deferens  to  the  female.  The  penis  of 
the  Crocodilia  and  of  the  Chelonia  is  feebly,  if  at  all, 
protractile,  but  an  advance  is  to  be  noted  in  the  greater 
union  of  the  two  halves,  and  the  development  of  blood 
spaces  or  corpora  cavernosa  in  its  substance.  Where 
a  penis  is  developed  in  birds,  it  has  (except  in  the 
ostrich)  the  form  of  a  coiled  protrusible  tube,  the 
groove  on  whose  upper  surface  leads  into  a  canal. 

Among  the  Eutheria  the  penis  always  presents  a 
certain  number  of  common  characters,  in  so  far  as  the 
outgrowth  of  the  cloaca  from  which  it  is  formed 
always  becomes  separated  by  the  perinseum  from 
the  rectal  orifice,  that  the  primitive  groove  always 
becomes  a  canal,  and  the  basal  or  proximal  end  is 
continuous  with  the  urogenital  sinus,  or  portion 
common  to  the  urethra  and  the  vasa  deferentia. 
This  penis  is  erected  by  a  median  and  two  lateral 
corpora  cavernosa,  and  at  the  free  end,  which  is 
covered  by  a  fold  of  the  skin  (prepuce),  there  is 
developed  a  glans:  In  addition  to  the  corpora,  an 
os  penis  may  be  found,  and  this  is  sometimes,  as 
in  the  walrus,  of  great  size.  The  portion  of  the 
urethra  found  in  the  penis  is  known  as  the  penial 
urethra,  and  it  sometimes,  though  by  no  means 
always,  traverses  the  body  of  the  glans  before  opening 
to  the  exterior. 


chap,  xiii.]      ACCESSORY  MALE  ORGANS.  521 

On  the  course  of  the  urethra  there  are  developed 
the  prostate,  and  frequently  also  the  Cowperian 

glands,  the  secretion  of  which  appears  to  serve  as  a 
vehicle  for  the  semen  ;  the  size  of  these,  and  espe- 
cially of  the  former,  varies  not  only  greatly  in 
various  species,  but  also  at  different  ages,  being 
often  much  larger  in  advanced  years.  The  glans 
itself  is  also  provided  with  preputial  glands,  and  is 
not  unfrequently  armed  with  spines.  In  Ornitho- 
rhynchus  it  has  a  kind  of  claw  at  its  end,  and  its 
surface  is  covered  with  spiny  hooks,  which  are 
directed  backwards,  and  serve  as  organs  of  attach- 
ment. 

The  penis  arises  as  an  outgrowth  of  the  wall  of 
the  cloaca,  and  is  at  first  grooved  along  its  upper 
surface  ;  the  sides  of  this  groove  grow  over  and  unite 
in  the  male.  On  either  side  there  is  a  fold  of  skin, 
which,  in  males,  unites  below  with  its  fellow  to  form 
the  scrotum,  in  which  the  testes  are  carried  in  a 
number  of  mammals.  In  the  female  the  just-men- 
tioned parts  remain  as  the  clitoris  and  labia  majora, 
and  do  not,  as  a  rule,  increase  greatly  in  size  ;  in 
some,  however,  like  the  hyaena,  they  are  as  large  as 
the  corresponding  or  homologous  parts  in  the  male, 
so  that  the  sexes  cannot,  externally,  be  distinguished 
from  one  another,  and  a  belief  in  the  hermaphroditism 
of  the  hyaena  is  consequently  not  uncommon.  The 
clitoris  is  perforated  by  the  urethra  in  some  of  the 
lemurs ;  and  in  the  seal  the  female  has  an  os 
clitoridis,  the  homologue  of  the  os  penis  of  the 
male. 

Internally  the  male  ordinarily  retains  a  small  por- 
tion only  of  the  Miillerian  duct.  This  uterus 
masculiiius  varies  considerably  in  size;  in  the 
rabbit,  for  example,  it  is  of  considerable  extent. 

Of  the  organs  connected  with  the  care  of  the  young, 
the  most  important  are  the  mammary  glands  and 


522  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


the  marsupial  pouch.  The  former  are  found  in 
all  the  Mammalia,  but  in  the  Monotremata  they  re- 
main at  a  low  or  indifferent  stage  in  so  far  as  the  ducts 
do  not  emerge  to  a  common  orifice,  but  open  separately, 


F 


Fig.  214. — Diagrams  of  the  Arrangements  of  th«  Ducts  of  the  Mam- 
mary Glands  in  various  Mammals. 

A,  Adult  echidna  ;  B,  human  embryo;  c,  adult  Homo ;  D,  adult  mouse;  E,  embryo 
of  Bos  ;  F,  adult  Bos.  (After  Klaatsch.) 

either  on  the  surface  of  the  integument  (Ornitho- 
rhynchus),  or  in  a  special  mammary  pouch  (Echidna) 
(Fig.  214;  A).  In  all  other  mammals,  "true"  or 
"  false  "  teats  are  developed,  the  former  being  found 
when,  as  in  man,  the  ducts  of  the  gland  converge  and 
open  on  an  uprising  projection  (Fig.  214 ;  c),  or  a 


Chap,  xni.]  MAMMARY  GLANDS.  523 

single  duct  (as  in  Mus ;  D)  traverses  the  projection. 
The  false  teat  is  found,  for  example,  in  the  cow  (T  and 
F),  where  the  sides  of  the  mammary  pouch  are  drawn 
out  to  form  a  canal,  at  the  pit  of  which  the  mammary 
ducts  open. 

Though  the  mammary  glands  are  merely  integu- 
mentary glands  set  apart  for  the  secretion  of  a  special 
fluid,  milk,  they  are  remarkable  for  being  unlike  any 
gland  found  in  any  other  vertebrate.  The  characters 
of  the  secretion  vary  within  limits  ;  the  milk  of  the 
mare,  for  example,  being  poor  in  fats,  but  so  rich  in 
sugar  as  to  easily  ferment,  and  form  a  spirituous 
liquor  (koumiss) ;  in  other  cases,  e.g.  the  goat,  the 
milk  has  the  odour  of  the  animal  that  secretes  it. 
As  a  rule  the  glands  are  active  in  females  only,  but 
medical  observers  have  put  on  record  a  few  cases  of 
"male  lactation." 

In  the  lower  Mammals  the  number  of  teats  is 
much  greater  than  in  the  higher  forms,  while  their 
presence  on  the  thorax  in  some  (man,  Sirenia),  and  011 
the  groin  (cow)  only  in  other  mammals,  speaks  to 
their  having  primitively  extended  along  a  large  part 
of  the  ventral  surface.  The  Centetidse  have  twelve 
pairs  of  teats  (Dobson),  the  rabbit  and  the  hedgehog 
five.  The  teats  are  often  found  to  correspond  in 
number  to  the  maximum  number  of  young  pro- 
duced at  a  birth ;  in  no  known  case  do  they  exceed 
fourteen. 

In  the  Marsupialia  the  teats  are  often  arranged  in 
a  circle  round  a  central  larger  teat,  and  the  whole 
mammary  area  is  enclosed  by  a  fold  of  the  skin,  the 
marsupial  pouch,  into  which  the  young,  which 
are  born  in  an  altogether  helpless  condition,  and  at  a 
period  so  early  that  they  are  unable  to  actively  suck 
the  mother,  are  conveyed.  These  helpless  babes  are 
fed  by  the  mother,  who  forces  the  milk  out  of  her 
mammary  glands  by  the  contraction  of  the  cremaster 


524  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

muscle ;  the  young  thus  fed  and  fixed  would,  no  doubt, 
soon  be  suffocated  were  their  respiratory  tube  of  ex- 
actly the  same  form  as  that  of  the  adult ;  to  prevent 
such  a  catastrophe  the  larynx  is  prolonged  at  its  upper 
end  into  a  conical  projection,  which  is  inserted  into 
the  cleft  of  the  soft  palate,  and  is  thereby  brought  into 
direct  connection  with  the  nasal  air  passages.  By 
these  means  the  nutrient  milk  can  pass  with  as  much 
safety  into  the  gullet  as  the  water  does  in  the  case  of 
the  Cetacea.  (See  page  242.)  Here  we  have  an  example 
of  how  in  different  groups  the  same  result  is  attained 
to  by  the  use  of  the  same  kind  of  means. 

Very  striking  differences  are  to  be  observed  among 
various  Vertebrates  as  to  their  care  of  their  young. 
Most  Fishes  lay  their  eggs  in  the  water,  where  they  are 
fertilised  by  the  male  and  left  to  hatch  ;  in  very  rare 
cases  does  the  female  exhibit  the  least  care  for  them ; 
indeed,  but  two  are  known ;  Aspredo,  after  deposit- 
ing her  eggs,  is  known  to  attach  them  to  her  belly, 
where  they  stay  till  they  are  hatched,  and  Solenostoma 
develops  a  pouch,  within  which  are  projecting  fila- 
ments to  which  possibly  the  ova  become  attached.  The 
male  more  frequently  exhibits  care  for  its  young,  the 
stickleback  and  others  forming  and  guarding  a  nest  in 
which  the  ova  are  deposited ;  others  carry  the  ova  in 
their  pharynx,  while  the  pipefish  (Syngnathus),  and 
the  seahorse  (Hippocampus)  develop  a  subcaudal 
pouch  in  which  the  ova  are  carried  till  they  are 
hatched.  The  female  Surinam  toad  carries  her  young 
on  her  back,  and  the  male  Alytes  obstetricans  cuts  the 
gelatinous  cords  by  means  of  which  the  ova  are  at- 
tached to  the  body  of  the  female,  and  twines  the  eggs 
round  his  legs.  Pythons  incubate  their  eggs,  and 
during  the  process  their  temperature  rises  about  3°  F. 
(Forbes).  Most  Birds  but  the  cuckoo  and  the  American 
Cow-bird  (Molothrus)  incubate  their  own  eggs,  and  in 
those  cases  where  the  young  are  unable,  when  hatched, 


chap,  xiv.]    DEVELOPMENT  OF  METAZOA.  525 

to  forage  for  themselves,  provide  them  with  food.  All 
Mammals  suckle  their  young,  and  the  ascent  in  the 
series  appropriately  finds  its  termination  in  man,  who 
alone  has  the  idea  of  the  family. 


CHAPTER   XIY. 

THE  DEVELOPMENT  OF  THE  METAZOA. 

THE  organs,  described  in  the  preceding  chapter, 
whether  essential,  as  the  testes  or  ovaries,  or  accessory, 
as  the  ducts,  glands,  and  intromittent  organs,  have  in 
common  the  function  of  providing  for  the  union  of  the 
male  cell  or  spermatozoon  and  the  female  cell  or  ovum. 
In  all  Metazoa,  save  those  which  multiply  by  budding 
or  by  the  parthenogenetic  development  of  the  egg, 
this  union  of  a  male  and  female  cell  is  the  first  step 
in  the  history  of  a  new  individual ;  it  may  be  effected 
either  within  the  body  of  the  female,  as  in  Birds  and 
Mammals,  or  it  may  be  effected  externally  to  it  as  in 
the  frog,  the  perch,  or  the  starfish.  The  ovum  having 
freed  itself  of  the  polar  globules,  and  received  within 
itself  the  male  or  fertilising  element,  proceeds  to 
undergo  division  or  segmentation  •  this  may  regularly 
affect  all  the  parts  of  the  egg,  and  the  several  seg- 
ments may  be  all  of  the  same  size,  or,  as  in  the  egg 
of  the  hen,  where  there  is  an  abundant  supply  of 
yolk,  division  may  go  on  actively  in  a  small  portion 
only  of  the  whole  egg.  It  is  particularly  to  be 
borne  in  mind  that  the  absence  or  presence  of  yolk 
in  small  or  larger  quantities  profoundly  affects  the 
character  of  the  segmentation. 

Regular  segmentation,  such  as  is  seen  in  Am- 
phioxus,  obtains  only  in  ova  in  which  there  is  none 
or  but  little  yolk  (aleoitlia I  ova ;  Lankester) ;  it  is 
most  common  among  the  lowest  Metazoa,  such  as  the 


526  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

Sponges  and  Coelenterata ;  it  is  found  in  some  round 
and  fiat  worms,  and  in  Echinoderms,  but  is  very  rare 
in  Insects,  Molluscs,  and  Chordates,  Amphioxus,  in- 
deed, being  the  only  representative  in  the  last- 
mentioned  phylum. 


Fig.  215.— The  Segmentation  of  Amphioxus. 

A,  Stage  with  two,  B,  with  four  equal  segment?.  In  c  each  of  the  four  segments 
has  been  divided  horizontally  ;  D,  a  later  stage  m  which  a  single  layer  of 
equal  cells  encloses  a  central  segmentation  cavity  ;  B,  the  cavity  (sg)  is  seen 
in  optical  section.  (After  Kowalevsky.) 


The  influence  of  food  yolk  on  the  segmentation  has 
been  stated  by  Balfour  in  the  following  terms  :  "  The 
rapidity  with  which  any  part  of  an  ovum  segments 
varies  ceteris  paribus  with  the  relative  amount  of 
protoplasm  it  contains  ;  and  the  size  of  the  segments 
formed  varies  inversely  to  the  relative  amount  of 


Chap,  xiv.i        HISTORY  OF  THE  OVUM.  527 

protoplasm.  When  the  proportion  of  protoplasm  in 
any  part  of  an  ovum  becomes  extremely  small,  seg- 
mentation does  not  occur  in  that  part." 

The  yolk  may  be  either  concentrated  in  the  centre 
of  the  egg  (centro-lecithal  ova)  as  in  many  Arthro- 
pods, or  at  one  pole  of  the  egg  (telo-Iecithal  ova), 
as  in  the  frog  or  the  chick.  In  the  former  case  the 
segments  may  be  all  equal,  or  some  may  be  larger  than 
others  (unequal  segmentation),  or  the  segmentation 
may  affect  only  the  surface  of  the  egg  ;  in  the  latter 
the  segmentation  may  be  unequal,  as  in  the  frog, 
where  there  is  not  a  large  amount  of  yolk  ;  or  partial, 
as  in  the  fowl,  where,  owing  to  the  enormous  quantity 
of  yolk  that  is  present,  segmentation  occurs  only  in 
the  small  area  to  which  the  protoplasm  is  confined  ; 
affording,  that  is,  a  proof  of  the  validity  of  the  law 
just  quoted. 

The  mass  of  cells  resulting  from  the  segmentation 
of  the  ovum  may  remain  closely  packed  together,  and 
resemble  a  mulberry  (morula  stage),  or  they  may 
separate  from  one  another,  and  give  rise  to  a  central 
segmentation  cavity  (Fig.  215  ;  E,  sg)  (folastula 
stage).  In  the  next  stage,  instead  of  a  single  we 
have  two  layers  of  cells,  and  this  condition  may  either 
be  brought  about  by  the  simple  inpushing  of  Borne 
of  the  cells  (invaginatioii),  or  some  of  the  cells 
may  grow  over  the  others,  or  the  cells  of  the  single 
layer  may  undergo  transverse  division  (delamina- 
f  ion),  and  the  arrangement  of  the  double  layer  be 
thus  arrived  at. 

As  we  have  already  learnt,  the  two-layered  stage 
with  a  central  cavity  is  known  as  the  Gastrula 
stage ;  the  layers  are  known  as  the  germinal  layers 
(epiblast  and  hypofolast),  and  the  cavity  as  the 
archenteron  (Fig.  216).  The  opening  into  the 
archenteron  is  known  as  the  folastopore,  and 
appears,  from  what  is  known  of  the  development  of 


528  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


such  primitive  forms  as  Peripatus  or  Amphioxus  to 
have  been  of  considerable  extent,  just  as  is  the 
mouth  of  a  sea-anemone  ;  like  it,  it  tended  to  close  up 
in  its  middle,  and  to  have  an  orifice  at  either  end 
(A.  Sedgwick)  ;  and  these  two  ends  would  therefore 
represent  the  oral  and  anal  orifices  of  the  primitive 
intestine.  The  fate  of  the  blastopore  is  very  various  ; 
it  may  remain  as  the  permanent  mouth,  or  as  the 

permanent  anus ; 
or  it  may  close 
up,  and  the 
mouth  or  the 
anus  be  subse- 
quently formed 
at  the  point  of 
closure,  or  it  may 
close  up  and  ap- 
parently have  no 
topographical  re- 
lation to  either 
the  future  mouth 
or  the  future 
anus. 

In  most, 
though  not  all 
(e.g.  Hydra)  Me- 

tazoa,  a  third  layer  (the  mesoblast)  appears  between 
the  epiblast  and  hypoblast,  and  gives  rise  to  the  sup- 
porting tissues,  the  muscles,  the  connective  tissue, 
and  the  blood  and  lymph  ;  the  epiblast  covers  the 
surface  of  the  animal,  and  is  the  seat  of  origin  of  the 
nervous  system  and  sense  organs,  the  hypoblast  lines 
the  alimentary  tract,  and  its  appended  glands.  As 
has  been  already  pointed  out,  a  cavity  (the  ccelom) 
appears  in  the  midst  of  the  mesoblast  (see  page  45) 
in  all  Metazoa  above  the  Ccelenterata. 

The  epiblast  becomes  divisible  in  all  forms  above 


Pig.  216.— Diagram  of  a  Gastrula. 
o,  Blastopore;  hyp, hypoblast ;  ep,  epiblast. 


Chap,  xiv.j  DEVELOPMENT.  529 

the  Coelenterata  into  two  layers,  the  epidermic  and  the 
nervous ;  the  former  gives  rise  to  a  structureless 
cuticle,  or  undergoes  hardening  and  becomes  chitinous, 
horny,  or  calcified,  thereby  giving  rise  to  the  various 
forms  of  external  skeletons  •  or  its  cells  may 
undergo  invagination,  and  give  rise  to  the  various 
forms  of  tegumentary  glands,  of  which  the  mammary 
are  the  most  specialised.  In  the  region  of  the  mouth 
the  epiblast  very  ordinarily  folds  inwards  to  form  the 
st  OHIO  <l;r  mil,  and  in  that  of  the  anus  to  form  the 
proctodseum. 

The  nervous  layer  undergoes  one  of  two  kinds  of 
modifications  ;  its  cells  either  become  thickened  along 
certain  tracts,  which  in  the  more  primitive  forms 
remain  permanently  connected  with  the  epidermic 
layer,  or,  as  in  all  the  Chordata,  the  nervous  layer  is 
grooved  along  its  middle  line,  and,  giving  rise  to  the 
medullary  canal,  becomes  separated  from  the  epider- 
mis. (See  page  416.)  Other  parts  of  the  layer  become 
modified  to  form  the  organs  of  sense. 

The  mesoblast,  the  mode  of  development  of  which 
varies  greatly  in  different  forms,  arises  in  such  primi- 
tive forms  as  Peripatus  or  Amphioxus  in  the  form  of 
paired  outgrowths  of  the  enteron ;  in  the  segmented 
Metazoa,  such  as  the  earthworm,  the  insect,  or  Am- 
phioxus, the  mesoblast  on  either  side  becomes  divided 
into  a  series  of  cubical  masses,  the  mesofolastic 
somites.  Where  the  mesoblast  arises  from  the  ente- 
ron, the  somites  are  from  the  first  hollow  within;  where, 
as  in  the  earthworm,  the  somites  arise  from  special  cells, 
and  are  at  first  solid,  a  cavity  is  gradually  developed 
within  them  ;  and  the  cavities  of  either  side  becoming 
in  time  continuous  with  one  another,  give  rise  to  the 
general  body  cavity  (ccelom).  The  outermost  portion 
of  the  mesoblast  comes  into  contact  with  the  epiblast, 
and  gives  rise  to  the  dermis  and  the  muscles  of  the 
body  wall ;  the  innermost  layer  comes  into  relation 


53°  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

\vith  the  hypoblast,  and  forms  the  greater  part  of  the 
wall  of  the  permanent  digestive  tract.  In  the  Verte- 
brata  the  upper  dorsal  portion  of  the  mesoblast 
surrounds  the  iiotochord,  and  gives  rise  to  the 
rudiments  of  the  bodies  of  the  vertebrae  ;  the  rest 
of  the  dorsal  portion  is  converted  into  the  so-called 
muscle  plates.  Other  parts  of  the  mesoblast  give  rise, 
in  various  ways,  to  the  heart  and  blood-vessels,  when 
such  are  present,  to  the  blood  itself,  and  to  the  renal 
and  generative  organs. 

The  hypoblast,  in  addition  to  forming  the  lining  of 
a  more  or  less  large  portion  of  the  digestive  tract,  and 
of  the  organs  (liver,  lungs)  which  are  developed  as 
outgrowths  from  it,  does,  in  the  Cephalochordata  and 
Urochordata  at  any  rate,  give  rise  also  to  the  noto- 
chord,  which  arises  as  a  diverticulurn  or  outgrowth 
on  the  dorsal  side  of  the  primitive  enteron. 

We  have,  then, 

(1)  The   epiblast   giving   rise    to   the    epidermis, 
nervous  system,  stomodaeum,  and  proctodaeum. 

(2)  The  mesoblast  gives  rise  to  the  internal  skeleton, 
the   muscles,   connective  tissue,   vascular,  renal,  and 
generative  systems. 

(3)  The  hypoblast  forming  the  lining  of  the  diges- 
tive tract  and  its  appendages,  and,  in  the  lower  Chor- 
data,  the  notochord. 

While  in  a  large  number  of  Metazoa  the  fertilised 
ovum,  by  a  regular  and  steady  series  of  differentia- 
tions, gives  rise  to  forms  which  essentially  resemble 
their  parents,  there  is  a  not  inconsiderable  number  of 
cases  in  which  the  young  live  for  a  time  an  inde- 
pendent life  under  a  form  very  different  to  that  of  the 
adult.  Such  forms  are  known  as  larvae ;  a  few  only 
can  be  here  dealt  with. 

Among  the  Cliordata  the  best  known  is  the 
tadpole  stage  of  the  frog  ;  instead  of  a  tailless  four- 
limbed  animal  with  a  large  mouth,  no  gills  or  .gill 


Chap.  XIV.] 


TADPOLES. 


clefts,  we  have  a  creature  with  a  long  tail,  a  sucking 
mouth  with  horny  jaws,  and  two  suckers  below,  and 
with  external  gills.  In  a  later  stage  of  metamorphosis 
the  gills  become  covered  by  an  operculum  (0)  \  two 
buds  between  the  body  and  the  tail  are  the  rudiments 
of  the  future  hind  limbs ;  later  on  the  signs  of  fore 


Fig.  217.— Structure  of  the  Tadpole. 

A,  From  the  side  :  B,  from  below  ;  c,  a  later  stage  ;  D,  head  of  tadpole ;  a  01  g2, 
gills ;  TO.  mouth  ;  j,  jaws  ; H,  nasal  sac  ;  e,  eye  ; «,  car ;  o,  operculum  ;  hi, rudimeu  t 
of  hind  limbs ;  s,  sucker ;  Ip,  upper  lip  ;  cP  to  cfi,  visceral  clefts. 

limbs  become  apparent,  the  gills  are  lost,  and  their 
clefts  disappear,  while  the  tail  gradually  undergoes 
atrophy,  and  the  larva  is  converted  into  a  small 
quadruped  tail-less  frog,  which  breathes  air  by  means 
of  lungs.  A  similarly  tailed  larval  stage  obtains  in 
some  of  the  Urochordata,  which,  in  adult  life,  are  fixed. 
Among  the  Arthropoda,  Insects,  as  we  well 
know,  present,  when  their  metamorphosis  is  "com- 
plete," three  distinct  stages ;  in  the  earliest  stage,  or 
that  of  the  "  larva"  (Fig.  218  ;  A),  the  product  of  the 


532   COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

developed  egg  has  a  more  or  less  worm-like  shape ;  it 
may  be  headless  and  legless  (maggot),  or  have  a 
head,  but  110  legs  (grub),  or  be  provided  with  head, 


Fig.  218.— A,  Larva ;  B,  Chrysalis ;  c,  Imago  of  Papilio  machaon. 


legs,  and  fore-legs  (caterpillar).  The  larva  grows, 
and  moults  its  skin  as  it  grows ;  after  a  time  it  ceases 
from  this  active  mode  of  life,  and  passes  into  a  more 
quiescent  condition,  as  in  the  case  of  the  butterfly,  or, 


chap,  xiv.]     LARVAL  STAGES  OF  INSECTS. 


533 


even  in  this  pupal  stage,  it  continues  to  move  about 
actively  ;  during  the  pupa  stage  a  number  of  changes 
occur  within  the  body,  and  organs,  such  as  the  wings, 
which  were  ab- 
sent from  the 
larva,  are  de- 
veloped from 
masses  of  indif- 
ferent cells,  the 
so-called  im- 
aginal  discs. 
The  most  com- 
plete series  of 
changes  during 
the  pupal  period 
obtain  in  the 
Flies;  all  the 
organs  of  the 
larva,except  the 
generative,  un- 
dergo degenera- 
tion, while  the 
abdomen  of  the 
imago  is  derived 
from  that  of  the 
larva;  the  imag- 
iiial  discs,  which 
are  formed  of 
minute  cells, 
and  enclosed  in 
a  structureless 

capsule,  grow  Fig.  219.— Larval  form  of  Cirripedia.  1,  Naxiplius  of 
T-orvIrlUT  4- "U™r>  Salanus  ;  2,  Larva  of  Chtliamalus  stellatv* ;  3,  Older 
rapidly  ,  \  Larva  of  Lepas  avftralis.  (After  Woodward.) 

in     the     lower 

portion  of  the  thorax  become  united  by  pairs,  and  give 
rise  to  the  legs  ;  those  in  the  upper  portion  become 
converted  into  the  wings  and  halteres  ;  tlie  cephalic 


534  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

discs    similarly   give   rise   to    the   head    and  its   ap- 
pendages. 

In  the  developmental  history  of  the  Crustacea 
there  are  two  larval  forms  or  stages  which  are  very 
widely  distributed  among  the  different  orders  ;  the 
appearance  of  these  has  been  of  very  considerable 
assistance  in  determining  the  real  zoological  position 
of  such  forms  as  the  barnacle  and  the  parasitic  Cope- 
poda,  which,  when  adult,  have  an  appearance  altogether 
unlike  that  of  Apus  or  Astacus  (Fig.  219). 

Like  many  other  larvae,  these  free-swimming  forms 
were,  when  first  observed,  thought  to  be  distinct 
animals,  and  received  in  consequence  distinctive 
names.  The  first  is  the  stage  known  as  that  of  the 
Nauplius.  In  this  the  larva  has  an  unsegmented 
body  and  three  pairs  of  appendages  of  which  the  two  pos- 
terior are  biramose,  a  single  median  eye,  and  a  distinct 
digestive  tract.  In  the  lowest  forms,  the  Phyllopoda, 
this  nauplius  passes  gradually  into  the  adult  stage,  the 
body  becoming  segmented,  and  fresh  appendages  ap- 
pearing as  the  crustacean  grows  in  size,  and  undergoes 
its  periodical  ecdyses,  or  sheddings  of  the  outer  skin. 

Among  the  higher  Crustacea  (Malacostraca)  the 
larvae  are  hardly  ever  found  freely  swimming  in  the 
Nauplius  stage ;  they  more  frequently  make  their 
appearance  at  a  more  advanced  period,  or  that  which 
is  known  as  the  Zoea.  Here  we  have  a  cephalo- 
thoracic  shield,  which  is  often,  though  not  always, 
provided  with  long  spiniform  processes,  the  longest  of 
which  projects  upwards  from  the  middle  of  the  back  ; 
the  tail  region  is  developed,  but,  like  the  hinder  part 
of  the  thorax,  it  is  without  the  appendages  that  are 
already  developed  in  the  anterior  region  of  the  body  ; 
lateral  eyes  are  present  in  addition  to  a  median  one. 
This  Zoea  stage  is  often  succeeded  by  others,  in  which 
certain  characters  are  greatly  exaggerated,  or  in  which 
there  are  presented  arrangements  which  are  permanent 


Chap.  XIV.] 


LARV/E  OF  CRINOIDS. 


535 


in  less  highly  developed  forms,  but  only  transitory  in 
the  higher ;  these,  however,  differ  in  different  orders, 
and  are  beyond  our  consideration  here. 

Finally,  it  is  to  be  borne  in  mind  that  some  Crus- 
tacea leave  the  egg  in  a  form  essentially  similar  to  that 
of  their  parent;  of  such  forms  the  crayfish  is  an  example. 

Some  remarkable  larval  forms  obtain  among  the 
Echinodcrmata,  and  the 
wide  distribution  of  species 
which,  when  adult,  are  capable 
of  but  a  slight  amount  of  loco- 
motion, must  be  ascribed  to 
their  possession  of  free-swim- 
ming ciliated  larvae.  The  most 
instructive  examples  are  pre- 
sented by  the  Comatulidse, 
which  are  members  of  the 
group  Pelmatozoa,  but  are 
stalked  in  their  larval  stages 
only,  during  which,  therefore, 
they  have  a  certain  resem- 
blance to  the  permanently- 
stalked  Pentacrinus.  After 
passing  through  a  short  period 
of  free  existence,  in  which  the 
cilia  are  arranged  in  four 
transverse  bands  (Fig.  220), 
and  during  which  two  sets  of 
five  plates  and  a  short  calcareous  stem  become  de- 
veloped, the  larva  loses  its  ciliated  bands,  and  becomes 
fixed  by  the  stalk  (Fig.  221 ;  A)  ;  at  the  free  end  of  this 
stalk  the  arms  become  developed,  and  below  the  cup- 
like  portion  (calyx)  there  appear  the  jointed  pro- 
cesses which  are  known  as  the  cirri.  The  calyx  and 
the  top  joint  of  the  stem  break  away  from  the  rest, 
and  we  get  the  Comatulid  which  is  capable  of  a  certain 
amount  of  locomotion. 


Fig.  220.—  Tortal  view  of  the 
Lnrva  of  the  Common 
British  Feather-star  (An- 
tedon  rosacea)  ;  x  20. 
(After  Wyville  -  Thom- 
son.) 


536  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Fig.  221.— Pentacrinoid  Larvse  of 
the  Feather  -  star  (Antedon 
rosacea). 

A,  Quite  young,  before  the  opening  of 
the  cup,  and  the  appearance  of 
the  five  radial  plates;  B,  nearly 
imture  ;  6,  basal ;  o,  orals  ;  r,  first 
radials.  (After  Carpenter. ) 


Among  the  Echinozoa 
we  often  find  arrangements 
which  exhibit  more  com- 
pletely the  characters  of  a 
true  metamorphosis,  and 
which  are  of  especial  inter- 
est because  they  present  a 
bilateral  symmetry,  such  as 
is  ordinarily  obscured  in  the 
adult.  The  simplest  con- 
ditions obtain  in  the  Holo- 
thurians.  After  passing 
through  the  early  stages 
of  development,  the  body, 
which  was  originally  co- 
vered with  cilia,  has  these 
processes  arranged  in  a 
sinuous  band  at  its  edges. 

The  anterior  portion  of 
the  enteric  tract,  before 
uniting  with  the  hinder  in- 
volution, the  orifice  of 
which  forms  the  permanent 
anus,  buds  off  a  vesicle, 
which  becomes  completely 
separated  from  the  enteric 
tract,  and  the  cavity  of  part 
of  which  forms  the  I>od3* 
cavity.  The  vesicle  elong- 
ates, and  sends  outwards 
a  process  which  comes  into 
contact  with  the  dorsal  sur- 
face of  the  body,  or  that 
which  is  opposite  to  the 
surface  on  which  the  mouth 
opens ;  this  process,  or  di- 
vert! julum,  has  an  opening 


Chap.  XIV.] 


LARV&  OF  ECHINOZOA. 


537 


to  the  exterior  (Fig.  222;  wp).    The  vesicle  then  breaks 

up  into  three  parts,  the  most  anterior  of  which  gives 

rise  to  the  water-vascular  ring  and  its  appended  canals, 

while  the  two  more  posterior  (Fig.  222 ;  rp,  Ip)  give  rise 

to   the    general    body 

cavity,    the    lining    of 

which    is    formed    by 

their  walls.     In  Holo- 

thurians     generally, 

though  not  always,  the 

connection  between  the 

vascular  system  and  the 

body    wall    becomes 

broken,  and  the  madre- 

poric  canal  hangs  freely 

in  the  body  cavity. 

Among  other  Echi- 
nozoa  the  amount  of 
difference  between  the 
larval  and  the  adult 
stage  is  much  greater 
than  it  is  in  Holothu- 
rians ;  the  larvae  are 
more  elaborately  de- 
veloped, and  present 
distinct  evidences  of 
secondary  adaptations 
to  their  free  mode  of 
life.  The  sides  of  the 
body  are  not  unfre- 

quently  produced  into  free  arm-like  processes,  the 
interior  of  which  may  (Pluteiis  larvae,  Fig.  223), 
or  may  not  (Brachiolaria),  be  supported  internally 
by  delicate  calcareous  rods.  Part  only  of  the  body  of 
such  larvae  passes  directly  into  the  substance  of  the 
adult ;  the  rest  is  either  -absorbed  by  the  growing 
echinozoon,  or  shrivels  up  and  disappears.  The 


Fig.  222.—  Diaenrammatic  View  of  the 
J  arva  of  a  Holothuriaa  (from  the 
side). 

m,  Mouth  ;  g,  gullet ;  s,  stomach ;  a,  anus ;  c, 
longitudinal  ciliated  band ;  w,  rudiment 
f  water-vascular  ring;  wp,  water-pore; 


rp,  Ip,  right  and  left  peritoneal  cavities, 
from  which  the  body  cavity  is  developed. 
(From  P.  H.  Carpenter,  after  Beleoka  ) 


538  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

connection  between  the  peritoneal  vesicle  and  the  outer 
world  is  permanently  retained,  and  forms  the  so-called 
madreporic  canal. 

A    few    Echinoderms    (Hemiaster,    Ophiacantha 
vivipara,  Chirodota  rotifera)  do  not  pass  through  any 


Fi.j  223.—  Pluteus  paradoxvs,  the  Larva  of  an  Ophiuroid,  at  a  late  stage, 
in  which  both  the  Larval  Arms  with  their  supports  and  the  rudi- 
ments of  the  Disc  and  Radial  Skeleton  of  Adult  are  to  be  seen. 
(After  J.  Miiller.) 

larval  stages ;  the  eggs  are  received  into  incubatory 
pouches,  or  are  developed  in  the  ccalom  without  pass- 
ing through  any  larval  stages,  or  leading  a  free- 
swimming  independent  existence. 

A  very  common  form  of  free-swimming  larva  is 
that  which  is  known  as  the  Trocliospliere,  and 
which  essentially  resembles  the  adult  condition  of  a 
Rotifer;  it  is  found  among  the  marine  Chsetopocla, 
some  of  the  Gephyrea  and  Mollusca,  and  in  the 


Chap,  xiv.j     THE  TROCHOSPHERE  LARVA.  539 

Bryozoa.  It  is  characterised  by  the  possession  of  a 
circlet  of  long  cilia,  which  separates  the  anterior 
portion  of  the  body  of  the  larva  (prseoral  lobe)  from 
that  which  lies  behind  it  (Fig.  224) ;  this  ciliated 
circlet  is  retained  throughout  life  by  the  Rotatoria. 
In  addition  to  it,  other  circlets  may  become  developed. 
The  most  common  of  these  is  that  which  appears  in 
the  region  of  the  anus  (telotrochal  larvae)  ;  in 
others  several  bands  of  cilia  are  formed  (polytrochal), 
and  these  sometimes  encircle  the  whole  body,  and  are 
sometimes  dorsal  and  some- 
times ventral  in  position. 
The  banded  condition  is 
preceded  by  one  in  which 
the  cilia  are  equally  distri- 
buted over  the  whole  body.  „  __ 

The  Trochosphere  is 
provided  with  a  definite  di- 
gestive tract,  the  lining  of 
which  is  ciliated :  has  a 

»   .  ,  ,,     ,         ,          ,  Fig.  224. — Larval  Chsetopods. 

fairly  well  developed  ner-  o>  Mouth .  anug .  ^  prffioral .  w>  post. 
vous  system  and  a  sensory  schLkC)Iliated  bandf  (After  Hat" 
apparatus  in  the  preeoral 

lobe ;  there  is  also  a  paired  excretory  organ,  which 
opens  into  the  body  cavity  by  several  funnel-shaped 
orifices.  As  the  postoral  portion  increases  in  length 
the  bands  of  mesoblastic  cells  undergo  segmentation, 
and  the  prseoral  portion  becomes  proportionately 
smaller.  Later  on  it  develops  the  tentacles  charac- 
teristic of  the  Chsetopod,  and  loses  the  band,  or  bands 
of  cilia. 

All  the  Mollusca  have  not  a  free-swimming  larva 
which  can  be  referred  to  this  type  ;  in  the  common 
fresh-water  mussel  the  ova  are  developed  under  the 
shelter  of  the  gills ;  here  they  become  provided  with  a 
bivalved  shell,  the  free  edges  of  which  are  toothed  ; 
the  larva  does  not  fix  itself  to  its  parent  by  these  hooks, 


540  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

but  by  the  byssus-threads,  which  are  secreted  by  a 
gland  at  the  hinder  end  of  the  body.  After  a  time 
the  Olochidium,  as  the  larva  at  this  stage  is  called, 
breaks  away  from  the  parent,  and  makes  it  way  to 
some  of  the  fish  that  live  in  the  same  water.  To  the 
gills  or  other  part  of  these  hosts  it  fixes  itself  by  its 
toothed  shell,  while  the  byssus  gland  becomes  aborted, 
as  do  also  the  sense  organs  with  which  the  larva  is 
provided.  Attached  to  and  covered  by  the  epidermis 
of  its  host,  the  young  mussel  undergoes  a  series  of 
further  changes  and  takes  on  the  characters  of  the 
adult. 

When  the  Molluscan  larva  is  referable  to  the 
trochosphere  type,  it  has,  as  Lankester  was  the  first 
to  point  out,  two  distinctive  characteristics  ;  on  the 
ventral  surface,  between  the  mouth  and  the  anus, 
there  is  a  projection  which  is  the  rudiment  of  the 
foot,  and  on  the  dorsal  surface  there  is  an  epiblastic 
ingrowth  which  forms  the  shell  gland.  The  larva  of 
Chiton  is  remarkable  for  having  the  posterior  dorsal 
region  segmented. 

The  simplest  of  all  known  larvae  are  found  in  the 
Coelenterata,  where  they  have  the  form  of  a  two- 
layered  oval  or  elongated  body,  covered  externally 
with  cilia,  and  provided  with  a  central  gastric  cavity, 
but  without  a  mouth.  In  the  simplest  cases  this 
Plan  11 1  a  becomes  fixed  by  one  end,  loses  its  cilia,  and 
begins  to  develop  tentacles  at  its  free  end.  In  the 
common  jelly-fish  (Aurelia)  and  in  the  vast  majority 
of  the  Acraspedota  a  very  remarkable  metamorphosis 
obtains.  The  free-swimming  planula  having  settled 
down  and  become  fixed  (Scyphistoma  stage)  in 
the  form  of  a  polyp  with  a  central  mouth  (Fig.  225 ;  A), 
begins  to  undergo  division  into  a  number  of  saucer- 
like  rings  set  one  below  the  other ;  each  of  these 
Strobila  contains  a  portion  of  the  gastric  cavity,  and, 
as  development  proceeds,  the  edges  of  the  saucers 


Chap,  xiv.j     DEVELOPMENT  OF  AURELIA. 


54i 


become  produced  into  eight  lobes  into  which  prolonga- 
tions of  the  central  cavity  extend.  After  undergoing 
some  further  development,  each  saucer  in  turn  breaks 
away  from  the  common  stock,  and,  as  an  Ephyra, 
with  a  disc  of  gelatinous  tissue,  a  layer  of  muscle, 
and  eight  bifid  tentacular  lobes,  swims  about  freely, 
increases  in  size,  and  becomes  gradually  converted 
into  an  adult  .sexually 
mature  jelly-fish.  Here, 
then,  we  have  an  ex- 
ample of  "alternation 
of  generations  " ;  the 
fertilised  ovum  gives 
rise,  through  the  plan- 
ula,  to  the  Hydra-tuba, 
the  parts  of  which 
undergo  by  constriction 
a  serial  multiplication, 
and  each  part  gives  rise 
to  a  sexually  mature 
form. 

/Alternation  of 
generations.  —  This 
complex  process  has, 
from  various  causes, 
been  considerably  ob- 
scured, and  various 

terms  have  been  applied  to  the  various  ways  in  which 
this  phenomenon  has  been  observed.  As  seen  among 
the  hydrozoic  Ccelenterates,  Annelid  worms,  and  Tuni- 
cata,  it  may  be  thus  described  in  the  words  of  Balfour  : 
"  The  simplest  cases  are  those  in  which  an  individual 
which  produces  by  sexual  means  gives  origin  to  asexual 
individuals  differently  organised  to  itself,  which  pro- 
duce, by  budding,  the  original  sexual  form,  and  so  com- 
plete a  cycle  ....  In  all  these  cases  the  origin  of 
the  phenomenon  is  easily  understood.  It  appears,  as  is 


Fig.  225.  —  Development  of  Aurelia 
aurita. 

A,  Polyp  stage ;  B,  commencement  of  trans- 
verse cleavage :  c,  completion  of  the 
same  so-called  Hydra-tube  stage.  (After 


542  COMPARATIVE  ANATOMI   AND  PHYSIOLOGY. 


most  clearly  shown  in  the  case  of  the  Annelida,  that 
the  ancestors  of  the  species  which  now  exhibit  alterna- 
tions of  generations  originally  reproduced  themselves 
at  the  same  time  both  sexually  and  by  budding,  though 
probably  the  two  modes  of  reproduction  did  not  take 
place  at  the  same  season.  Gradually  a  differentiation 
became  established,  by  which  sexual  reproduction  be- 
came confined  to  certain 
individuals,  which  in 
most  instances  did  not 
also  reproduce  asexu- 
ally.  After  the  two 
modes  of  reproduction 
became  confined  to  se- 
parate individuals,  the 
dissimilarity  in  habits 
of  life  necessitated  by 
their  diverse  functions 
caused  a  difference  in 
their  organisation ;  and 
thus  a  complete  alter- 
nation of  generations 
became  established. 
The  above  is  no  merely 
speculative  history, 
since  all  gradations  be- 
tween complete  alternations  of  generations  and  simple 
budding  combined  with  sexual  reproduction  can  be 
traced  in  actually  existing  forms." 

When  alternation  of  generations  is  fully  expressed 
among  the  Hydrozoa  we  find  that  the  sessile  hydri- 
form  colony  gives  rise  to  buds  which  gradually  break 
away  from  their  colony  and  become  free-swimming 
(Fig.  226).  Differing  in  some  details  from  the 
structure  of  the  Medusa  already  noted,  these  forms  are 
still  more  interesting  in  that  between  them  and  the 
ordinary  hydroid  polyp  we  find  a  series  of  stages  which 


Fig.  226. — Figure  of  Syncoryne  with  a 
numV>er  of  Budding  Medusae  on  it 
at  Different  Stages  (a  to  e)  of  De- 
velopment. (After  Desor.) 


Chap,  xiv.]    ALTERNATION  OF  GENERATIONS.          543 

have  been  variously  regarded  as  grades  of  development 
or  oi  degradation. 

We  find,  that  is,  that  the  medusiforni  buds  do  not 
always  become  separated  from  the  stock  that  has 
produced  them ;  and  while  in  some  cases  (e.g.  Syn- 
coryne  itself,  towards  the  end  of  the  breeding  season, 
or  Tubularia)  the  buds  are  fully  formed  medusae,  in 
others,  though  still  bearing  the  sexual  organs,  they 
are  nothing  more  than  projections  from  the  sides  of 
the  body,  in  which  the  medusoid  characters  are  hardly, 
if  at  all,  apparent  (Hydractinia).  These  stages  of 
difference  in  the  medusoid  buds  are  allied,  011  the  one 
hand,  to  the  condition  which  obtains  in  the  common 
Hydra,  where  ova  and  spermatozoa  are  developed  in 
one  and  the  same  individual,  and  in  which  the  young 
do  not  pass  through  any  larval  stage ;  and  on  the 
other,  to  what  is  seen  in  Geryonia,  for  example,  where 
the  hydriform  condition  isaltogether  suppressed,  and  the 
larva,  after  a  certain  amount  of  metamorphosis,  passes 
into  the  medusoid  condition  of  its  parent.  In  both  of 
these  cases  there  is  no  alternation  of  generations. 

A  series  of  very  interesting  conditions  are  exhi- 
bited by  different  Annelids.  In  Lumbriculus  there 
may  be  simple  transverse  division  of  the  body,  one 
half  of  which  acquires  a  new  tail,  and  the  other  a  new 
head ;  in  Ctenodrilus  it  has  been  observed  that  the 
anterior  half  of  the  body  may  again  divide ;  in  Syllis 
the  generative  products  are  developed  in  the  posterior 
half  only  of  the  body  ;  in  Myrianida  the  same  pro- 
ducts are  confined  to  the  forms  that  arise  by  budding, 
so  that  from  a  simple  case  of  transverse  division  we 
have  come  to  a  complete  example  of  alternation  of 
generations. 

In  some  cases  (e.g.  the  fresh-water  Nais)  there  is 
not  simple  transverse  division,  but  the  formation  first 
of  all  of  a  so-called  "  zone  of  gemmation ; "  here  the 
zone  becomes  converted  anteriorly  into  an  anal  zone, 


544  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

and  posteriorly  into  a  fresli  head  ;  several  zones  of 
gemmation  may  appear  before  the  zooids  break  away 
from  the  parent,  and  begin  to  develop  generative 
organs.  In  Protula,  the  parent  reproduces  sexually, 
as  well  as  the  buds,  but  in  Autolytus  the  genital 
glands  are  confined  to  the  zooids  that  have  been  de- 
veloped by  budding. 

The  most  complicated  alternations  are  found  among 
the  Urochordata,*  a  large  number  of  which  multi- 
ply by  budding ;  a  simple  case  is  presented  by  those 
forms  in  which  the  bud  arises  as  an  outgrowth  of  the 
body  wall,  together  with  a  prolongation  of  part  of  the 
intestine.  From  this  outgrowth  the  organs  of  the 
bud  are  fashioned,  and  the  bud,  breaking  away,  gives 
rise  to  fresh  buds.  Both  bud  and  parent  develop 
generative  organs  and  reproduce  themselves  sexually. 

In  Botryllus  the  product  of  a  fertilised  ovum  gives 
rise  to  a  single  bud ;  this  gives  rise  to  two,  each  of 
which  again  develops  two  buds ;  the  four  buds  ar- 
range themselves  round  a  common  cloaca,  then  give 
rise  to  two  or  three  buds,  and  these  again  to  others. 
These  last,  which  may  go  on  budding,  are  the  first  that 
are  provided  with  sexual  organs. 

In  Pyrosoma  the  product  of  a  fertilised  ovum  gives 
rise,  while  still  an  embryo,  to  four  zooids  ;  these  re- 
produce sexually,  and  so  give  rise  to  fresh  colonies,  or 
multiply  by  budding,  and  so  increase  the  size  of  the 
colony. 

The  height  of  complexity  is  reached  by  Doliolum, 
the  embryo  of  which  is  at  first  tailed,  but  becomes 
cask-shaped  in  form,  like  its  parent.  From  its  dorsal 
surface  there  grows  out  a  process  or  Stolon,  at  the 
sides  and  along  the  dorsal  middle  line  of  which  buds 
appear.  The  former  become  converted  into  the  spoon- 
like  forms  of  Gegenbaur,  and  become  free ;  their 

*  The  account  given  by  Balfour  ("  Comparative  Embryology," 
voL  ii.)  has  been  closely  followed  here. 


Chap,  xiv.]       DEVELOPMENT  OF  T^ENIA.  545 

further  history  is  as  yet  unknown.  The  dorsal  buds 
take  on  the  form  of  the  parent  with  sexual  organs, 
but  do  not  themselves  become  sexually  mature  ;  they 
develop  a  stolon  from  their  ventral  surface,  on 
which  appear  buds  that  grow  up  into  the  sexual 
forms. 

The  relations  of  these  different  stages  is  shown  by 
the  following  table  : 

Sexual  generation. 

First  asexual  form  with  dorsal  stolon. 


Spoon-like  forms  developed        Second  asexual  forms  developed  as 
as    lateral   buda   (future          median  buds  with  ventral  stolon, 
history  unknown).  | 

Sexual  generation. 

A  somewhat  different  condition  of  things  is  found 
among  the  endo-parasitic  forms,  where,  as  a  rule, 
the  animal  passes  through  its  different  stages  in  two 
different  hosts  ;  we  may  take  as  typical  the  histories 
of  the  common  tapeworm,  and  of  the  liver-fluke  which 
causes  the  "  rot  "  in  sheep  (Distomum  hepaticum). 

Taenia  solium  is  sexually  mature  in  the  intes- 
tine of  man,  and  the  final  joints  of  the  tapeworm 
consist  merely  of  fertilised  ova,  which  have  al- 
ready passed  through  the  earlier  stages  of  develop- 
ment ;  when  the  joints  become  free  and  escape  to  the 
exterior,  they  break  up,  and  the  contents  escape  in 
the  form  of  embryos  contained  in  a  thick  chitiiious 
shell.  If  these  are  now  swallowed  by  a  pig,  the 
shell  is  digested  by  the  gastric  juices  of  the  new  host, 
and  a  rounded  embryo,  which  is  provided  with  three 
pairs  of  hooks,  is  set  free ;  by  means  of  these  hooks 
the  guest  makes  its  way  through  the  wall  of  the 
stomach  or  intestine,  and  finally  settles  down  in  the 
muscles  of  its  host.  The  embryo  now  loses  its  hooks 
and  gradually  acquires  a  bladder-like  form,  the  central 
cavity  of  which  is  filled  with  fluid,  while  circular  and 
JJ— 16 


546  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 

longitudinal  muscular  fibres  are  developed  in  its  walls. 
This  bladder- worm  (cysticercus),  now  has  its  outer 
wall  pushed  inwards  at  the  anterior  end,  and  on  the 
involution  so  formed  hooks  and  suckers  become  de- 
veloped, in  such  a  way  that  when,  as  next  happens,  the 
involution  is  turned  inside  out,  these  hooks  and  suckers 
lie  on  the  outer  surface  of  the  so-called  "head." 

We  have  now  a  narrow  head  and  neck  with  an 
attached  bladder  (Fig.  227),  the  head  being    at    this 
time    hollow,    and  having    in    it    a 
circular  vessel  which  communicates 
with  four  longitudinal  fibres. 

If,  during  the  long  time  that 
these  "  bladder-worms  "  remain  alive, 
the  pig  is  killed  for  food,  and  after- 
wards insufficiently  cooked,  they  are, 
when  the  pork  is  eaten,  conveyed  into 

Fig.227.— Cyshcercus      ,,          ,  J 

cellulose  (after Von    the     human    stomach.       Here    the 

fhfnSd  ($f*eX    ^adder-like  termination  becomes  ab- 

(c),  and  Vesicle  (a),     sorbed,  and  the   neck,  increasing  in 

length,  becomes  divided  into  joints 

which  are  constantly  produced  at  the  anterior  end  ; 

the  oldest  joints  (proglottids)    are,  in  other  words, 

farthest  from  the  head.     In  them  sexual  organs  are 

developed,  and  the  cycle  recommences. 

Distomum  hepaticiim,  of  which  several  hun- 
dreds may  occupy  the  liver  of  one  sheep,  is  of  extra- 
ordinary fecundity,  producing  at  least  as  many  as  one 
hundred  thousand  ova  ;  these  only  pass  through  their 
earliest  segmentation  phases  in  the  warmth  of  the  mam- 
malian body,  but  when  they  escape  and  reach  a 
moderately  warm  and  moist  place,  the  egg  commences 
to  develop  rapidly  within  its  firm  shell.  When  ready 
to  escape  as  an  elongated  ciliated  larva,  the  embryo 
bursts  the  cap  of  its  shell,  and  begins  to  move  about 
freely.  If  the  pasture  on  which  it  has  fallen  is  moist, 
the  larva  soon  finds  a  stream  of  water  along  which 


Chap,  xiv.]       DEVELOPMENT  OF  FLUKE. 


547 


it  may  pass  to  the  neighbourhood  of  its  next  host ; 
this  has  been  shown  by  Thomas  to  be  the  small  air- 
breathing  snail  which  is  known  as  r.ymiiseiis 
trimcatuliis.  Provided  at  its  anterior  end  with  a 
papilla  which  acts  as  a  most  effi- 
cient boring-organ,  the  larva  forces 
its  way  between  the  cells  of  the 
wall  of  the  lung  of  the  Lymnseus, 
and  makes  its  way  into  the  lung 
cavity.  In  this  position  it  loses  its 
elongated  and  acquires  a  rounded 
form,  giving  rise  to  the  so-called 
sporocyst  stage.  The  cells  with- 
in the  body  which  have  not  yet 
been  used  up  in  the  formation  of 
any  tissue,  arrange  themselves  in 
definite  groups,  each  of  which  gives 
rise  to  an  elongated  larval  form 
not  unlike  a  gastrula  (Fig.  228), 
save  that  it  is  provided  with  a 
definite  pharynx,  has  an  "  annular 
ridge,"  and  two  short  blunt  pro- 
cesses behind.  We  have  now  the 
Redia  stage.  The  R6dia,  be- 
coming free,  may  make  its  way 
into  other  organs  of  the  snail's 
body ;  within  this  Redia  fresh 
Redise  may  be  again  developed,  or 
the  germinal  cells  within  it  may,  in-  Fig._228.  —  Redia  of  D. 
stead,  give  rise  to  yet  another  form. 
At  any  rate,  the  final  product 
of  redise,  or  daughter-rediae,  is  a 
body  of  rounded  form  with  a  long  tail  (Fig.  229), 
to  which  the  name  of  cercaria  has  been  long  since 
applied.  The  parasite  in  this  stage  makes  its  way 
to  the  exterior,  and,  becoming  enclosed  in  a  firm  cyst, 
loses  its  tail;  these  cercarian  cysts  take  up  their 


Hepaticum. 
Thomas.) 


(After 


n,  Pharynx  ;  m,  contained 
germs  ;  r,  posterior  pro- 
cesses. 


548  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


position  at  the  roots  of  the  grass,  and  so  on,  and  in 
time  either  die  down,  or  are  eaten  by  a  sheep.  When 
the  latter  misfortune  happens,  they  pass  into  the 
stomach,  and  so  to  the  gall  ducts  and  liver,  to  grow 
up  afresh  into  the  likeness  of 
the  liver-fluke  from  which  they 
started. 

Histories  not  unlike  those  of 
these  two  divisions  of  the  Platy- 
helminthes  are  presented  by  the 
round  •  worms  or  Nematoliel- 
mintlies,  and  by  the  Echino- 
rliyiiclii.  The  thread  -  worm 
of  the  human  blood  (Filaria 
sanguinis  hominis),  which  appears 
to  be  the  cause  of  chyluria  and 
of  some  other  diseases  in  the 
countries  of  the  Eastern  Old 
World,  has  been  found  to  have 
an  intermediate  host  in  the 
mosquito,  from  whom  it  passes 
into  water ;  when  this  water  is 
drunk  the  young  return  to  the 
human  intestine.  Dracunculus 
medinensis  lives  in  its  adult  con- 
dition in  the  subcutaneous  tissue 
of  the  human  leg  and  foot :  its 
larval  stages  being  passed,  as  it 
seems,  in  a  fresh- water  crustacean. 
Trichina  is  an  example  of  a  form 
which  appears  to  have  had  its  history  modified  ;  in 
societies  that  may  be  called  cannibal  (e.g.  rats)  no  in- 
termediate host  would  appear  to  be  necessary  ;  in 
the  case  of  civilised  man,  the  adult  worms  are  obtained 
from  the  flesh  of  incompletely-cooked  pigs. 


Fig.  229.— Cefcaria  of  D. 
hepaticum.  (After 
Thomas.) 


INDEX. 


Acanthocephala,  51 ;  hooks,  28? ; 

gonads,  489 

Acanthometra,  27 ;  skeleton,  276 
Acarina,  74 ;  respiration,  228 
Achseta ;  cilia,  56 
Achtheres ;  gnathites,  179 
Acineta,  31 ;  suckers,  177 
Acipenser,  91 ;  skull,  326 
Acraspedota,  42  ;  nervous  system, 

395 

Actinophrys,  27 
JErobranchiata,  73 
Alcippe,  69 

Amblystoma ;  oral  glands,  158 
Amia,  92 

Ammothoa ;  figure,  72 
A.mniota,  93 
Amoeba  ;  structure,  18 ;  zoological 

position,   27 ;   respiration,  210  ; 

ectosarc,  274 
Amphibia,    89,    93;   teeth,    145; 

tongue,  154  ;  oral  glands,   158  ; 

heart,  195,  202 ;  respiration,  232, 

236  ;    kidneys,  259 ;  glands,  266 ; 

vertebral      column,     314,    321 ; 

skull,  330;  mouth,  336 ;   scales, 

365  ;  brain,  421 ;  sensor v  organs, 

436,  454,  467  ;  gonads,  506 
Amphioxus  ;  segmentation,  32,  87 
Amphipoda,  70 
Amphiuma,  93 ;  blood  corpuscles, 

Amphonyx ;  proboscis,  132 
Ampullaria ;  respiration,  228 
Anabas,  93  ;  suprabranchial  organ, 

234 
Anguillulidffi  ;    oral  bristles,  114 ; 

respiration,  211 
Anisopleura,  81 
Annulata,     54;     digestion,     117; 

gills,  218;  skeleton,  286;    eyes, 

446  ;  gonads,  489 
Anodon,  80  ;  blood-vessels,  191. 
Anoplophrya ;  figure,  104 
^.ntedon,    63;    nervous    system, 

409 ;  copulation,  492 

jj*— 16 


Anthozoa,  43 ;  mouth,  110 ;  sipho- 
noglyphe,  111 ;  figure,  112 ; 
yellow  cells,  272 ;  skeleton,  280 ; 
muscles,  373 ;  gonads,  484 

Anthropoidea,  100 

Anura ;  respiration,  236  ;  kidneys, 
259 ;  testes,  506 

Aphrodite,  54;  cseca,  118 

Aplysia,  81 ;  intestine,  137 

Appendicularia,  87  ;  heart,  193 ; 
house,  313 

Aptera ;  zoological  position,  75  ; 
gnathites,  128  ;  springs,  379 

Apus ;  nervous  system,  405 ;  ova, 
494 ;  carapace,  299 

Arachnida ;  organisation,  72 ;  para- 
sitic, 180;  heart,  190;  respira- 
tion, 225  ;  limbs,  377  ;  gonads,  499 

Area ;  gills,  220 

Arenicola;  gills,  219 

Argon auta;  shell,  309 

Argulus ;  guathites,  180 

Arthropoda,  64;  gnathites,  122; 
blood-vessels,  187 ;  tracheae,  215 ; 
stigmata,  216  ;  renal  organ,  256; 
skeleton,  291;  locomotion,  375; 
nervous  system,  405 ;  sensory 
organs,  434,  438,  418,  461 ;  go- 
nads, 494 

Artiodactyla,  99  ;  limbs,  357 

Ascaris,  51 

Ascefcta ;  figure.  35 

Ascidia,  88  ;  atriopore,  231 

Ascon,  35 

Asiphoniata,  80 

Aspidogaster,  49 

Asterias,  63 ;  figure  of  arm,  61  ; 
skeleton,  294 

Astropecten,  63  ;  figure,  59  ;  loss 
of  anus,  121 

Atlanta,  82 

Aurelia,  42 ;  figure,  42  ;  respira- 
tion, 211  ;  gelatinous  tissue, 
286  ;  nervous  system,  395 

Axolotl  ;  oral  glands,  158 

Azygobranchiata,  82 


53°  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Balanoglossus  ;  figure,  86 

Balanus,  69 

Balistes  ;  teeth,  144 

Bdellostoma ;  kidneys,  259 

Beroe  ;  mouth,  113 

Birds,  97;  tongue,  154;  oral 
glands,  159 ;  crop,  162  ;  gizz.ird, 
163 ;  intestine,- 169  ;  bursa  f a- 
hricii,  170 ;  vitelline  duct,  171  ; 
arterial  arches,  205  ;  lungs,  239  • 
kidneys,  261;  metallic  colours 
273;  vertebrae,  316,  321;  jaws 
340 ;  furcula,  349  ;  limbs,  352 
feathers,  366;  wings,  336  ;  voice 
390;  brain,  423;  spinal  cod 
430 ;  sensory  organs,  437,  442, 
455,  470  ;  gonads,  506 

Boltenia,  88 

Bonellia ;  proboscis,  118 

Bothriocephalus ;    joints,  50  ;   go- 
nads, 455 

Botryllus,  88 ;  development,  544 

Brachionus,  52 

Brachiopoda,  100  ;  shell,  311 

Branchippoda,  67  j  figure,  66  ;  re- 
spiration, 223 

Brissopsis  ;  nervous  system,  399 

Bryozoa,  101 ;  intestine,  120 ;  cell, 
288 ;  gonads,  489 

Buccinum,  82 

Bugula ;  figure.  101 

Butiriuus,  IK) ;  intestine,  168 

Caducichordata,   87 ;    sen»e-cells, 

431 

Casciliee,  93  ;  penis,  520 
Calcispongiee,  35 
Caligus  ;  gnathites,  179 
Camels  ;  water-bag,  167 ;  foot,  357 
Cardium,  80 
Carinella,    85:     nervous    system, 

398 

Carmarina ;  nervous  system,  396 
Carnivora,  99  ;  jaws,  342 
Caryophyllseus.'SO 
Centetes;  lower  jaw,  342;  teats, 

523 

Centrogouida ;  organisation,  69 
Cephalochordata,    86 ;     digestive 

tract,  138;  vascular  system,193; 

respiration,    231 ;    renal  organ, 

257 ;  gonads,  506 

Cephalopoda  ;  ink-bag,  138  ;  bran- 
chial   hearts,    192;    gills,   221; 

funnel,   222  ;  renal   organ,   255 ; 

ear,  463 ;  gonads,  503 
Ceratodus  ;  teeth,  144;  heart,  195, 

202 ;  fin,  362 
Ceratospongias,  36 ;  skeleton,  278 


Cestoda,  49;  nitrogenous  waste, 
250  ;  hooks,  287  ;  gonads,  485 

Cestus,  45  ;  figure,  46 

Cetacea,  99;  teeth,  whale-bone,153  ; 
salivary  glands,  160 ;  retia  mi- 
rabilia,  209  ;  respiration,  242; 
kidneys,  281 ;  skull,  344  ;  tail, 
381  ' 

Cetochilus ;  figure,  68 

Chalina,  36 

Chamselpon  ;  tongue,  154 ;  chro- 
matophore-',  27 '£ 

Chsetoderinatidae,  81 

Chsetodon,  93  ;  teeth,  144 

Chsetognatha,  101 

Chelonia,  96 ;  fins,  97 ;  intestine, 
169;  bursse  auales,  170;  fin, 
353,  384.  S«e  Tortoise. 

Chirocentrus,  90 ;  intestine,  168 

Chiroptera,  99 

Chitonidee,  81 ;  shell,  306  ;  eyes,  457 

Chordata,  86 ;  digestive  tract, 
138 ;  vascular  system,  192 ;  re- 
spiration, 230;  skeleton,  312; 
nervous  system,  415;  eye,  452; 
gonads,  505  ;  larvae,  530 

Cicada  :  vocal  organ, 389 

Ciduris,  63 

Ciliata,  30 

Cirripedia,  69  ;  shell,  299,  304 

Clione,  82 

Clypeaster,  63 

Cockroach ;  zoological  position, 
76 ;  figure,  76  ;  gnathites,  130  ; 
intestine,  133  ;  gonads,  497 

Coelenterata,  36, 40  ;  trophosomes, 
109  •  gastro-vascular  canals,  185  ; 
respiration,  211 ;  nitrogenous 
waste,  248  ;  gonads,  484.  See 
also  Medusse. 

Coleoptera ;  characters,  77 ; 
gnathites,  131;  stomach,  133; 
elytra,  298 ;  vocal  organ,  389 

Copepoda,  68 

Corallinm  ;  coral,  282 

Cordylophora,  40 

Crania ;  figure,  100 

Craspedota,  40 

Crayfish  ;  mouth  organs,  123  ;  gas- 
tric mill,  125  ;  heart,  187 ;  re- 
spiration, 223;  scaphognathite, 
123,  225;  green  gland,  253; 
skeleton,  301  ;  nervous  system, 
413;  sensory  organs,  439,  449, 
451 ;  gonads ;  495 

Crinoidea;  organisation,  58;  di- 
gestion in,  121  ;  skeleton,  292  ; 
nervous  system,  408;  gonads, 


INDEX. 


55' 


Crocodile,  97  ;  stomach,  163  ;  ver- 
tebra, 315;  skull,  340;  scales, 
355 

Crotalus  ;  skull,  338. 

Crustacea";  organisation,  65 ; 
gnathites,  128  ;  respiration,  223 ; 
branchial  formula,  224  ;  nitro- 
genous waste,  253;  skeleton, 
298 ;  locomotor  organs,  376 ; 
gonads,  499  ;  larvse,  534 

Cryptocarpa,  40 

(Jryptoniscidse ;  gonads,  500 

Ctenophora,  45;  cilia,  tentacles, 
373;  gastric  cavity,  113 

Cucunmria,  64  ;  lungs,  229 

Cyamus,  72 

Cyclodus,  96  ;  scales,  366 

Cyclops  ;  zoological  position,  68 ; 
figure,  68  ;  gnathites,  179 

Cyclostomata,  88 ;  mouth,  140 ; 
gills,  223 ;  kidney,  258  ;  skeleton, 
314,  328 ;  gonads,  506 

Cyrnbulia,  82 

Cymothoa ;  hermaphroditism,  500 

Cyprcea,  82 

Cytozoa,  24 

Demodex,  180 

Dendroccelum,  49 

Dentalium  ;  figure,  82 

Desmodus;  stomach,  167 

Didelphia,  99 

Diodon,  92  ;  exoskeleton,  385 

Diphyes,  41 

Dipnoi,  90  ;  heart,  195  ;  pulmo- 
nary vessels,  203  ;  lungs,  i35 

Diptera,  77 ;  guathites,  132  ;  sto- 
mach, 134  ;  imaginal  disks, 
533 

Distomum,  49;  digestion,  114; 
gonads,  486 

Dog  ;  tongue,  157  ;  salivary  glands, 
160 

Dogfish  ;  teeth,  141 ;  fins,  360 

Dolium,  82  ;  salivary  glands,  137 

Dolphin ;  f ore-limb,  354 

Doris,  81 

Draco,  96  :  flying  organs,  383 

Dytiscus ;  eye,  450 

Earthworm  ;  digestion  in,  115  ; 
typhlosole,  117;  blood,  183; 
spermatozoa,  479  ;  gouads,  490 

Echidna,  98 ;  teats,  522 

Echinanthus,  63 

Echinodennata,  58  ;  digestion  in, 
120 ;  respiration,  218,  229  ;  renal 
waste,  248 ;  skeleton,  288  ;  pedi- 
cellariae,  297,  374;  movements, 


374 ;    nervous  system,  400,  407 ; 

sensory  organs,  434, 435, 444, 461 ; 

gonads,  491 ;  development,  535 
Echinoidea ;      organisation,      63 ; 

skeleton,  289  ;  sphaeridia,  435 
Echinometra,  63 
Echinorhynchus  ;  figure  50 
Echinozoa,  63 
Echinus  ;  zoological  position,  63  ; 

test,  290  ;  nervous  system,  408  ; 

gonads,  491 
Edentata,  99 
Elasmobranchs,  90 ;  heart,  195  ; 

labial     cartilages,    335;    brain, 

420  ;  taste-organs,  436  :  gonads, 

506 ;  placenta,  514 
Elephant ;  molars,  154  ;  skull,  343 
Entoconcha,  180 
tntomostraca ;     parasitic,      179 ; 

blood  -  vessels,    188  ;    skeleton, 

298 

Ephemeridse  ;  gnathites,  133 
Ericulus;  kidneys,  264 
Euplectella ;  zoological  position, 

36;  skeleton,  279 
Eurystomata,  96 
Euspongia ;    zoological   position, 

36 

Eutheria,  99.     See  Mammalia 
Euthyneura,  81 
Exocoetus,  93 ;  figure,  382 

Fierasfer,  181 

Filaria,  51 

Firuloides,  82 

Fishes ;  teeth,  142 ;  spiral  valve, 
168 ;  heart,  201 ;  circulus  cepha- 
licus,  205;  gills,  hair-bladder, 
232;  operculum,  233;  kidneys, 
258;  poison  glands,  266;  eye- 
like  spots,  269;  vertebrae,  314; 
skeleton,  323;  fins,  359;  bran- 
chial bars,  328;  mouth,  336; 
sounds,  392  ;  brain,  420 ;  spinal 
cord,  430 ;  sensory  organs,  436, 
440,  453,  466;  eye-like  organs, 
458  ;  gonads,  506  ;  oviducts,  513 ; 
copulation,  519 ;  care  of  young, 
524 

Fbgellata,  30 

Fluke  ;  digestion,  178 ;  nephridia, 
248 ;  gonads,  406 

Fowl ;  pelvis,  351 

Frog,  93;  heart,  197;  carotid 
gland,  203 ;  vertebras,  315 ;  skull, 
330;  brain,  419;  gonads,  506 

Galago ;  tongue,  157 
Galeopithecus ;  flight,  385 


552  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY. 


Ganoidei,  90 ;  snout,  336 

Gastropoda;  divisions,  80;  diges- 
tive tract,  135;  blood-vessels, 
191  ;  gills,  221 ;  lungs,  228 ;  re- 
nal organ,  255 ;  foot,  380  ;  go- 
nads,  502 

Gephyrea,  56 ;  digestion.  118 ;  re- 
spiration, 230 ;  nephrida,  251 ; 
gonads,  489 

Glossophora,  80  ;  odontophore,  135 

Gnathopoda,  -58 

Gnathostomata,  88  ;  mouth,  140 

Goose ;  gizzard,  164 

Gordius,  51  ;  mouth,  114,  179 

Gorgonia ;  zoological  position, 
44  ;  figure,  44  ;  skeleton,  281 

Grantia,  36 ;  sense-cells,  431 

Gregarina,  25 ;   reproduction,  474 

Gromia,  27  ;  figure,  26  ;  test,  275 

Gymnosomata,  82 

Gymnotus  ;  electric  organs,  269 

G>mnnra;  teetb,  150;  stomach,  165 

Heematobranchiata,  73 

Haliotis,  81 

HaHs.rca,  35 

Hatteria,  96  ;  teeth,  146 ;  vertebrae, 
315 ;  iaws,  339 

Haustellata,  77 

Hedgehog  ;  teeth,  151 

Hedriophthalmata,  70 

Helicidse  ;  teeth,  134  :  gonads,  502 

Heliozoa,  27,  29;  skeleton,  276 

Helix,  81 

Heloderuia  ;  oral  glands,  159 

Hemiptera,  77 ;  gnathites,  132 

Hesione  ;  air  sacs,  230 

Hesperornis,  98 

Heteropoda,  82 

Hirudinea,  56  ;  sensory  organs, 
433  ;  gonads,  489 

Histozoa,  24 ;  nephridia,  252 

Holopus,  121 

Holothuria,  64 ;  lungs,  229  ;  cuvie- 
rian  organs,  268  ;  skeleton,  296  ; 
larva,  372  ;  gonads,  492 

Homo,  100;  erect  position,  356; 
brain,  429;  scrotum,  507;  ovi- 
ducts, 519  ;  teats,  522 

Horse ;  teeth,  153 ;  stomach,  165  ; 
foot,  355 

Hyalea,  82 

Hy  alone  ma  ;  spicules,  279 

Hydra;  figure,  36,  40;  digest'ou, 
105;  chlorophyll,  272  >  sense- 
cells,  432 ;  gonads,  484 

Hydractinia,  40 

Hydrocorallinse,  40 

HydromedussD,  40 ;  coral,  283 


Hymempteri,  77;  gnathites,  130; 

vocal  organ,  389 
Hyomoschus ;  foot,  357 
Hyrax,  99  ;  caeca,  172 

Ichthyopsida,  89 

Infusoria,  29  ;  digestion,  103  ;  re- 
spiration, 210  ;  lorica,  278 ;  cilia, 
371  ;  reproduction,  476 

Insecta;  gnathites,  128;  enterou, 
133  ;  salivary  glands,  Malpighian 
vessels,  134  ;  heart,  190  ;  respi- 
ration, 216  ;  poison  glands,  266  ; 
silk  glands,  567 ;  appendages, 
303  ;  wings,  378 ;  vocal  organs, 
388 ;  nervous  system,  406 ;  sen- 
sory organs,  436,  449,  439,  461 ; 
gonads,  497,  500 ;  larvae,  531 

Insectivora,  99  ;  brain,  426 

Isopleura,  81 

Isopoda,  70  ;  parasitic,  180  ;  respi- 
ration, 229  ;  gonads,  499 

Lacerta,  96 

Lacertilia,  96  ;  oral  glands,  159 
Lremodipoda,  72 
Laganum,  63 

Lamellibranchiata,  79  ;  gills,  219  ; 
glochidia,  221 ;  renal  organ,  254  ; 
byssus,  268 ;  shell,  306 ;  foot, 
380 ;  nervous  system,  410 ;  eyes, 
457  ;  ear,  462  ;  gonads,  501 
Lancelet.  Sen  Cephalochordata 

Leech  ;  mouth,  179  ;  respiration, 
212  ;  nephridia,  252 ;  movements, 
373 

Lenviroidea,  100 

Lepidoptera,  77;  gnathites,  131; 
stomach,  134 

Lepidosiren,  90  ;  teeth,  144 

Lepidosteus,  92  ;  teeth,  144  ;  air 
sac,  235  ;  ear,  465 

Lepisma,  75 

Leucon,  36 

Ligula,  50 

Limax,  81 ;  shell,  305 

Limulus,  70;  respiration,  225; 
carapace,  299;  eye,  451;  sper- 
matozoa, 499 

Linckia,  63 ;  comet  form,  494 

Lineus,  85 

Lion ;  skeleton,  318 

Lipobranchiata,  74 

Lipocephala,  79 

Lizard;  heart,  204;  arterial  ar- 
ches, 205 ;  gonads,  512 

Loligo,  83;  shell,  309;  spermato- 
phore,  505 

Lophius ;  teeth,  142 


INDEX. 


553 


Lumbricus,  56.    See  Earthworm. 
Lymnseus,  81 ;  lung,  228 

Mactra,  80 

Malacostraca,  69 

Mallophaga;  gnathites,  133 

Mammalia,  98 ;  teeth,  148 ;  tongue, 
157  ;  salivary  glands,  159 ;  sto- 
mach, 165 ;  intestine,  170  ;  caeca, 
171 ;  liver,  173 ;  blood-corpuscles, 
182  ;  heart,  198 ;  arterial  arches, 
205 ;  jugulars,  208 ;  lungs,  240, 
246 ;  kidney,  261 ;  bladder,  263 ; 
vertebral  column,  316 ;  skull, 
332 ;  jaws,  342  ;  hairs,  368 ;  nails, 
331 ;  flying  organs,  383  ;  larynx, 
392 ;  brain,  425 ;  spinal  cord, 
430;  sensory  organs,  437,  443, 
456,  471  ;  gonads,  507 ;  placenta, 
514  ;  vagina,  517  ;  penis,  520  ; 
teats,  521 

Mandibulata,  76 

Manis,  99  ;  vertebrae,  317 

Macrolyristes ;  vocal  organ,  388 

Marsupials ;  respiration,  243 ; 
bladder,  263 ;  lower  jaw,  342 

Medusae ;  gastro-vascular  canals, 
110 ;  nervous  system,  395 ;  eyes, 
444 ;  ear,  459  ;  gonads,  484 

Meuobranchus,  93  ;  gills,  236 

Menopoma,  93 ;  gill  clefts,  236 

Mesostomum,  49 ;  digestion  in,  107 

Metatheria,  99 

Metazou,  31 

Millepora,  40 ;  coral,  283 

Mole ;  sternum,  348 

Mollusca,  77  ;  odoutophore,  134 ; 
parasitic,  180;  intestine,  sali- 
vary glands,  136;  heart,  191  ; 
respiration,  219  ;  lungs,  228 ; 
renal  organ,  254;  byssus,  268; 
skull,  304;  foot,  380;  nervous 
system,  410 ;  sensory  organs, 
439,  447.  449  ;  gouads,  501 

Monera,  27 

Monocrelis ;  nephridia,  248 

Monodelphia,  99 

Mouotremata  ;  retia  mirabilia, 
209  ;  bladder,  263 

Musk-deer  ;  feet,  355 ;  brain,  427  j 
cloaca,  517 

Mussel ;  gills,  219 ;  organ  of 
Bojanus,  255  ;  byssus,  268 

Mya ;  figure,  80 

Myodora,  79 

Myriopoda,  74 ;  gnathites,  127 ; 
heart,  189 ;  respiration,  216 ; 
appendages,  377  :  gonads,  499 

Myxiue,  88,  181 


M.yxospongiae,  35  ;  ova,  483 
Myzostomum,  102 

Nais,  56 

Nautilus;   figure,   83;    gills,  221; 

shell,  308;  funnel.  381 
Nematoids,    50  ;     digestion,    114, 

178;   respiration,   211;    gouads, 

488 
Nemertinea,     84 ;    blood-vessels, 

186 ;  respiration,  213 ;  gonads,  489 
Neomeuiidae,  81 
Nepa ;  gnathites,  132 
Nereis,  54 
Neuroptera,  77 ;   gnathites,    131 ; 

stomach,  133 
Notacanthus,  92 
Nudibranchs  ;  respiration,  222 
Nummulites,  27,  28  ;  test,  276 

Oceania.  40 
Octactiuiae,  43 
Odontopteryx ;  jaw,  146 
Odoutornithes,  98  ;  teeth,  146 
Oligocheeta,  56 
Opalina  ;  digestion  in,  104 
Ophideres;  proboscis,  132 
Ophidia ;    vertebrae,    314 ;    skull, 

337 ;  scales,  355  ;  oviducts,  513 
Ophiocephalus,  92 
Ophiocoma,  63 
Ophiothrix,  63 
Ophiuroidea,       63  ;      respiration, 

218  ;  skeleton,  295  ;  gonads,  491 
Orchesella,  75 
Ornithodelphia,  98 
Oruithorhynchus,  98  ;  heart,  198  ; 

pelvic  arch,  350 
Orthoptera,  76;    guathites,    129; 

vocal  organs,  388 
Orycteropus,  99 
Ostracoda,  69 
Oyster,  79;  foot,  380;  gonads,  501 

Paludina,  82  ;  gill,  228 

Pangonia  ;  proboscis,  132 

Paramoacium ;  figure,  29 

Parrots  ;  tongue,  157  ;  skull,  340 

Patella,  81 ;  gill,  221 

Peachia  ;  mouth  of,  112 

Peccary ;  stomach,  165 

Pedipalpi,  74 

Pelmatozoa,  63 

Peltogaster,  69 

Pennatula,  70 ;  phosphorescence, 
270 

Pentacrinus ;  figure,  60,  63  ;  skele- 
ton, 293 

Peutastomum ;  hooks,  180 


554  COMPARATIVE  ANATOMY  AND  PHYSIOLOGY* 


Perennichordata,  87 

Perigonimus ;  figure,  37 

Peripatus;  figure,  64:  gnathites, 
127;  blood-vessels,  189;  tra- 
chese,  215 ;  nephridia,  257 ;  skin, 
299  ;  appendages,  302,  377 ;  ner- 
vous system,  401;  sensory  or- 
gans, 434,  417  ;  gouads,  498 

Perissodactyla,  99 

Petronayzon,  88 

Pkolas,  80 

Physalia,  41 

Pbysoklisti,  92 

Physophora  ;  figure,  41 

Physostomi,  92 

Pipa ;  tongue,  153 

Plinaria,  49 

Platyhehnintb.es,  49 ;  gonads,  485 

Pueumodermon,  82 

Podophthalmata,  70 

Podura,  75 

Polia,  85 

Polychseta,  54 ;  larva,  539 

Polynoe,  54 

Polyodon,  91 

Polyplacophora,  81 

Polypterus,  92 ;  brain,  421 

Porifera,  35 

Primates,  100 ;  limbs,  356 

Pristis ;  teeth,  142 

Proneomenia ;  nervous  system, 
401 ;  gouads,  502 

Proteus,  93;  gill,  236 

Prototheria,  98 ;  vertebrae,  317 

Protozoa,  25 ;  digestion,  103 ; 
parasitic,  177 ;  contractile  va- 
cuoles,  247 ;  phosphorescence, 
270  ;  movements,  371 ;  repro- 
duction, 473 ;  skeleton,  274 

Protracheata,  65 

Pteropoda,  81 

Pycnogonoidea,  74 

Pyrophorus ;  phosphorescence,  270 

Pyrosoma.  88;  phosphorescence, 
270 

R  .bbit ;  caecum,  172  ;  brain,  42 1 ; 
taste-bulbs,  438  ;  placenta,  515 

Radiolaria;  yellow  cells,  272; 
skeleton,  276 

Ratitse,  97  ;  feathers,  368 

Reptiles,  96 ;  teeth,  145 ;  salivary 
glands,  159;  oesophagus,  162'; 
intestine,  169 ;  heart,  196  ;  lungs, 
239  ;  respiration,  245 ;  kidneys, 
261 ;  brain,  422 ;  sensory  organs, 
437,  454,  467,  471 ;  gonads,  512 

Rhizocephala,  69 

Rhizocrmus,  63 


Rhizopoda ;  skeleton,  276 

Rhyncopygus ;  anus,  121 

Rodents,  99 ;  teeth,  150 

Rotatoria,  50  ;  digestion,  118  ;  re- 
spiration, 230 ;  nephridia,  251 

Ruminants ;  stomach,  166  ;  retia, 
209 

Sabella,  55 

Sacculina,  69;  nutrition,  178 

Sagartia,  39  ;  section  of,  111 

Salamaudra,  93  ;  ribs,  345 

Salmon ;  teeth,  144 

Salpa,  88 

Sargus ;  teeth,  142 

Sarsia,  40 

Sauropsida,  95  ;  uric  acid,  265  ; 
vertebral  column,  314,  321 ; 
skull,  331 

Scaphopoda,  82 

Scarus ;  teeth,  142 

Scorpion;  gnathites,  127  ;  mouth, 
179 ;  lung-books,  pectiues,  226 ; 
poison,  266 

Sea-gull;  gizzard,  176 

Selachoidei,  91 

Sepia,  83 

Serpula,  55 

Siuupalliata,  80 

Siphonophora,  41 

Sirenia,  99;  respiration,  2 13;  ver- 
tebrae, 317;  limbs,  353 

Snakes ;  teeth,  146 ;  arterial  ar- 
ches, 205 

Solen,  80 ;  haemoglobin,  212 

Spitangidse,  63 

Spide/ ;  poison  glands,  265 ;  silk 
organs,  267 

Sponge,  31  ;  digestion,  106  ;  re- 
spiration, 211;  nitrogenous 
waste,  248 ;  skeleton,  278 ;  sense - 
cells,  431 ;  gonads,  483 

Squatina  ;  heart,  196 

Squilla ;  figure,  69 ;  testis,  498 

Steatornis ;  syrinx,  391 

Steganophthalmata,  42 

Stenostomata,  96 

Stentor,  30 ;  stentorin,  272 

Strepsiptera,  77 

Streptopeura,  81 

Stylasteridae ;  dactylozooids,  109 

Suctoria,  31 ;  digestion  in,  104 

Sun-birds  ;  tongue,  156 

Sycon,  36 

Synapta,  64  ;  spicules,  296 

Taenia,  49  ;  digestion  in,  177 ; 
plasmatic  canals,  185 ;  develop- 
ment, 545 


INDEX. 


555 


Teleostei,  90  ;  teeth,  142 

Teleostoidei,  92 

Terebella ;  figure,  55 

Termitidaj,  77  ;  gnathites,  133 

Tetrabranchiata,  82 

Thecpsomata,  82 

Theriomorpha,  95 

Torpedo;  electric  organs,  269 

Tortoise  ;  shell,  3t>6  ;  brain,  4-2 

Toxotes,  93 

Tracheata,  74 

Tragulus  ;  blood  corpuscles,  182 

Trichina,  50,  548 

Trichoptera,  77  ;  gnathites,  131 

Tridacna,  80^  shell,  308 

Triton,  93  ;  shell,  307 

Tubicolse,  55;  setae,  288 

Tubifex,  56 

Tubipora  :  figure,  43  ;  skeleton, 
283 

Tunicata,  86 ;  eye,  452  ;  ear,  464 

Tupaia ;  brain,  425 

Turbellaria  ;  digestion,  107,  113  ; 
circulation,  185 ;  respiration, 
211 ;  nephridium,  249 ;  nervous 
system,  400 ;  tactile  organs,  433 ; 
eyes,  445 

Turtle  ;  heart,  197 ;  skeleton,  320 ; 
skull,  339 

Typhlops,  96 


Ungulata,  99 ;  feet,  355 
Urochordata,  86;  digestive  tract, 


139 ;  vascular  system,  192 ;  re- 
spiration, 231 ;  renal  organ,  257  ; 
uotochord,  313 ;  larva,  544.  Sea 
Tunicata. 

Urodela,  93;  respiratioa,  236; 
kidneys,  259 ;  gonads,  506 

Uropeltis,  96 

Vagantia,  55 

Velella,  41 

Vertebra ta,  88;  digestive  tract, 
140;  teeth,  141;  tongue,  157; 
intestine,  161 ;  liver,  175 ;  blood, 
181 ;  vessels,  194 ;  retia  mirabilia, 
208  ;  haemoglobin,  212 ;  respira- 
tion, 231 ;  renal  organ,  257 ; 
vertebral  column,  314;  skull, 
323;  limbs,  347;  vocal  organ, 
390;  brain,  417;  spinal  cord, 
430;  sensory  organs,  435,  437, 
440,  453,  463 ;  gonads,  505 

Viper ;  fang,  147 

Vorticellids,  30;  contractile  va- 
cuoles,  247  ;  movements,  371 

Whales  ;  see  Cetacea,  99 

"Wolf;  teeth,  152 

"Woodpecker;  tongue,  156;  skull, 

Xiphacantha ;  figure,28 ;  skeleton, 
Zygobranchiata,  81 


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Renewed  books  are  subject  to  immediate  recall. 


Ja14'58FL 


MAY  2  2  1959 


BMY'5  IDF 


